It’s completely normal, it just sucks. This happens to me all of the time, and I always do on-color feedings and in groups of ten. No TH or 3*+. If you think about it, there’s only a 1%-30% chance that it will even go up, and that’s not all that high. Now, if it’s at 100% and still not going up, then that would definitely be a bug. Anything less than that can and will miss, more often than not.
If it’s a 3* that’s not getting it’s skill up, then that’s normal. It might get stuck at 3/8.
4* rarely get to max without 8/8, but it has happened.
5* I don’t think have ever not been 8/8 by last ascension.
Avoid feeding trainer and 3*+ heroes to anyone who doesn’t have a maxed skill. Try different methods; on-color feeding, rainbow feeding, one at a time, ten at a time, six at a time. Keep plugging away and hopefully eventually it will come.