Learn from it, and use what you have learned. I honestly killed what would have been at least 1,000 angels years ago learning how to hatch, feed, and raise one successful spawn. It took me a few months of pulling spawns & killing fish to get it right.
Keep a log of everything you do, sometimes the smallest things make the biggest difference. Change plenty of water frequently, and consider bare bottom tanks for raising fry. Years ago I thought bare tanks were ugly as sin, now I look at a tank with substrate and think one thing; filth.
You are not alone in the loosing fry department, one in a while I lose nearly all or all of an entire spawn for no reason. I review my log for that spawn, once in a while it's a procedural error, often caused by work & the sudden amounts of massive overtime causing me to skip water changes. Other times it happens for no apparent reason, and the next spawn is totally fine. I think this situation is similar to people who can have several totally perfect children, and one with medical or physical problems, that's the way nature works at times.
A friend of mine breeds guppys, has been doing it for 30 years. 35 tanks with auto water changers, a really nice setup. He has been working towards a strain of 100% orange or red guppys that breed true for a few years now. 6 weeks ago he turned on the auto water changer, right afterward his wife needed something for the yard from a shelf above the water mix valves. He grabbed this large jug, accidentally turned off the hot water, didn't realize it, and went outside for a half hour. He came back in to several hundred dead fry & breeders in water that was 50F. It happens to the best.