The hoya with a hollow stem is most likely dead. All live stems have tissue in the center. Dead ones are hollow.

All hoyas produce sap. It can range in color from clear (carnosa) to yellow green (caudata) to white (pottsii).

The hoya that has indentations in the leaves is most likely Hoya carnosa 'Chelsea' or Hoya carnosa 'Krinkle 8'. It is normal.

Now, to the health of your plants:

Your potting mix looks bad. Hoyas need a very well draining mix, such as fine grade orchid mix. If you are going to use a soil and perlite mix, you have to cut back on watering, as soil retains A LOT of moisture, which can cause root rot. You also must change a soil mix yearly, as it compacts over time, causing more drainage issues.

Check your roots for rot and change your potting mix.

Also, almost all hoyas prefer being rootbound (potbound), meaning they prefer smaller pots. Once you see roots coming out the drainage holes in your pots and coming out above the top of the pot, you can repot. Repot in a pot 1-2 inches larger than the old pot.

When is the last time you FED your hoyas? All potted plants need fertilizer. That is how they get nutrients to grow properly. If your hoyas are not fed, they get yellow leaves, fail to grow, and decline in health. Hoyas should be fed a water soluble fertilizer at 1/2 strength every watering. I use MiracleGro water soluble tomato plant food (1/2 tsp per gallon of water) every watering. It is cheap, and has all needed nutrients and micronutrients. Alternatively, you can use a time released, balanced food, like Osmocote.

Hoyas also do just fine in more light. As a matter of fact, most hoyas prefer bright, filtered sun, such as a West facing window with a sheer curtain or sheer blinds. The exception would be your Hoya polyneura (2nd to last photo), which would do just fine in your East window.

Thanks aurorawa(8B) for your great advice.

To confirm - change to a orchid mix, would you suggest repotting these plants in smaller pots when I change the mix?

Plants were fed with a Seaweed solution about a month ago. When they were repotted about 2 months ago a slow release fertiliser and soil conditioner were added to the mix.

All plants are have always been outside in the elements with morning sun till 11am - we are getting into Summer now.

I also have a more mature Hoya Plant that I would like some advice on, please see pictures below. I will repot this plant in an orchid mix as suggested above and also more regular fertiliser feeding.

Thanks.

Okay, I did not realize you were on the opposite side of the world! That makes a difference, regarding the very light leaf color. Sun exposure can also cause light colored leaves. Put your hoyas in less direct sun, and they will green back up.

Yes, change to an orchid mix. If you are using a slow release fertilizer, no need to feed during watering, but be sure to add more slow release when it is due (check package, some are every month, some last 3-6 months).

Regarding the small hoyas, yes, move to smaller pots, if you have them. If not, it's not too big a deal, as long as your potting mix is well-draining. Check the roots for signs of rot when repotting. Rot causes the rooting stem (where roots come out) to turn black or brown and squishy. The roots will look stringy, brown or black in color and might smell sour or bad.

For your polyneura (2nd to last photo in your first post), I suggest moving that one inside, as it likes cooler conditions. It also prefers to stay evenly damp (not soaked or soggy).

Regarding your large hoya, that one spot is sunburn. The pitting leaves can be due to sucking bugs (check for aphids, scale or mealybugs), or they could just be natural mutant leaves (it happens). If it is bugs, treat with a sharp dpray of water or, of you don't mind chemical warfare you can treat with systemic insecticide, like this:

https://www.amazon.com/Compare-N-Save-Systemic-Shrub-Insect-Drench/dp/B00ARKS5QO/ref=sr_1_2?ie=UTF8&qid=1475654521&sr=8-2&keywords=compare+and+save+insect+control

Alternatively, if you'd rather use a gentle and bee-friendly formula:

2 tablespoons of baby shampoo

1/4 cup rubbing (isopropyl alcohol) alcohol

Put in spray/spritz bottle. Fill rest of bottle with water.

Spray entire surface of plant. KEEP OUT OF BRIGHT LIGHT/SUNLIGHT until plant is completely dry. Repeat as necessary, until no more pests are seen.

As far as trellising that plant, you may want to, as they can get pretty heavy, which can cause breakage. Easy solution:

Small twist ties or orchid clips can be used to train those vines without the use of a trellis! Just wrap the vines around and up the hanging pot and secure onto the hanger part with ties or clips, like this:

Hoya calycina clipped to hanging pot:

Keep new growth/vine tips pointed toward the sky, to prevent vine dieback.

That large plant looks healthy, tho. Don't think I would be changing the mix it is growing in unless there is a reason. ( the old saying...."If it ain't broke...don't fix it".) can be good advise. The big white spot does look like sunburn. Trellising or hanging of most hoyas is really just a matter of preferrance, altho some are better "climbers" than others. Most are happy to do either. They don't like the young tender vines clipped too tightly or pointed downward however.