What kind of white rain lilies are they? Zephyranthes candidum (rio de la plata rain lily)? I have hill country rain lily (Zephyranthes drummondii) and black prairie rain lily (Zephyranthes chlorosolen)both of which are white and I could trade those two natives of Texas for yours. I also have pink pampa rain lily (Habranthus robusta)to trade. I could also trade oxblood lilies (Rhodophiala bifida) and red chinese spider lily (Lycoris radiata).

I do not personally grow either of these, but they are both bulbs, right? The vast majority of bulbs grow, bloom, and then "ripen" their foliage. You know it's ripening when it starts to turn yellow, then brown, then "die". It's not really dead, though. The bulb is just going dormant. It will reappear to do its thing again next year.

Basal rot perhaps? Symptoms are premature yellowing and dying of the stalk. Dig up the bulbs on the cut down or fallen lilies and inspect the bulbs. Or - are you sure about the drainage? At this point, you've got nothing to lose by digging the affected plants up and taking a look at them.

Why not post a few more pictures including one of the soil and one of the whole plant, and tell us how you water and what kind of light the plant gets? I suggest you find out exactly what's wrong before doing anything else. Fortunately there are a lot of people here who have lots of experience raising all sorts of plants and dealing with just about any nasty thing that can harm them. I have a few guesses, but a little more information will help folks figure out what's going on and the best way to treat it. Fungus gnats live in the soil, so you have to do something to the soil to get rid of them. I treat them by periodically getting all of my plants to dry out at the same time, because they can't survive without wet soil. I've heard of people getting good results by dissolving part of a mosquito dunk in water and then watering lightly with it. I'm sure someone will pop in and give you some details. Often, the kind of wilting and yellowing leaves I see in your picture are signs of watering issues, and usually it's overwatering. Many people don't believe it's possible, but watering while the soil at the bottom of the pot is still wet keeps the rootlets that carry water to the plant from breathing and ends up killing them, making it look like the plant is thirsty when the problem is drowned roots. I can tell my plants are starting to get dry from their weight, but I check the soil moisture daily with a sharpened wooden dowel (just switched from a cheap moisture meter) stuck all the way to the bottom of the pot, and don't water until the first time the tip feels dry. At that point there's still moisture in the soil, but if I wait any longer the plant will wilt. At that point I water thoroughly until a lot of water has drained from the bottom of the pot and let the pot drain thoroughly. There are ways of helping this along, but we can get to that later. Best of luck!