I have tried several times to propagate my African violet plant from leaves (both in water and in soil), but every time, the main leaf dies before the new leaves are viable enough to grow on their own. How do I stop this from happening?

I propagate all of my AV leaves in water. Have a special container that hangs in the kitchen window-facing west. Some leaves produce tiny leaves early in the rooting process and some don't. When I get a good sized group of roots I pot the leaf, with or without the little leaves. I think the ones with the tiny leaves progress a bit faster, but I don't remember not having a leaf develop a new plant. I have been known to get as many as three plants from one leaf. When it is well settled in its pot, and growing, I cut the leaf stem off near the surface and put it back in the water. Bill

I start my AV leaves in vermiculite. The leaf already has all the nutrients it needs to grow roots and shoots. Vermiculite has practically no nutrient value, and therefore no nutrient value to harbor disease, either, and naturally provides a semi sterile environment so the AV leaf can grow through the vulnerable stage without problems.

Put the moistened vermiculite in your container. Plant the leaf. Water to settle the vericulite. Let the pot drain for at least an hour. Put the pot on some paper towels and watch more water come out. Seal it up in a freezer bag, put in bright light but no direct sun. No need to do anything more until you see sprouting.

This is just the way I do it. There are many other successful ways, too.