The odds are the problem will be found to be one of 3 things. A) A combination of a water-retentive soil and over-watering B) A high level of soluble salts in the soil resulting from watering in small sips so most or all of the solubles from fertilizers and tap water accumulate in the soil C) Tight roots The tree is beginning to get the poodle look, where foliage tends to be concentrated near apices (growing tips of branches), but the burning on leaf margins suggests over-watering or a high salt concentration in the soil. The logical thing to do would be to repot (as opposed to potting up), making sure you get the plant into a soil that allows you to water properly. Fortunately, you're coming into the best time of the year to perform that chore. Feeling ambitious? Al

Looks like the plant is in the process of shedding the leaf. Do check for the possibility of spider mites. The lesions on the underside probably represent the natural progression of a reaction to particular cultural conditions that cause something called oedema. Al

If I'd give it a lot more light. What type is soil is it in? Did you repot or pot up? lastly, I'd place my bet on overwatering and accumulated salts in the soil. Watering once a week by just feeling the top of the soil means it's probably still very damp down below. Watering with a a cup also ensures you're not flushing out the soil regularly. Get a wooden dowel and sharpen it. Stick it in the soil all the way to the bottom and then pull it out. If it's dark and damp don't water. Wait until it comes out clean and dry to water. You can keep checking every couple days.

What, exactly did you use and how did you apply it? It looks like you're seriously over-watering, in spite of the moisture meter. They don't work well because they actually don't measure moisture, they measure electrical conductivity. To see what I'm saying is true, clean the tip and insert it into a half cup of distilled water in a squeaky clean cup. It'll read DRY. Add one drop of synthetic fertilizer, which adds ions, which increases electrical conductivity, which makes the 'dry' water WET ...... according to the moisture meter. Better to use a tell. I'll leave something I wrote about using a "tell", before I go. Forgive the wonky format that Houzz provides frr whenever you paste anything here. Also, how you described the light where the plant is sited isn't inspiring. Ficus tend to readily shed their leaves when their chemical messengers tell plant central, Hey - we're not getting enough light!" This leads not only to shedding of foliage, but other issues that affect the plant's growth rate, appearance, and general well being. There is something called a LCP, or Light Compensation Point, which is that point at which carbohydrates required by plants in respiration are equal to carbohydrates produced by photosynthesis - essentially the 'break even' point. Photosynthetic rate must equal respiration rate if the plant is to survive. Plants languishing in limbo are almost always presenting one problem or another, so I would strongly encourage you to think about additional light. Using a 'tell' Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. Al