I noticed that the ascencion skills books are not working in upgrading skill but only in skill training. Why?

On each upgrade icon, you see a number, that number indicates which level your skil need to reack to unlock that upgrade ability.
If you see "1" upgrade in upgrade is available on first level
If you see "2" upgrade is available on second level, and you need 3 same skillbooks to unlock lv 2
If you see "3" its available on third level, and you need 6 more same skillbooks to upgrade from lv 2 to lv 3.

Hope this clarifies to you.

I know that there is a percentage for every skills books to upgrade and that is not the problem. There are ascension skill books in my inventory but when I presss the upgrade skill it is not working. It only works when the skill books are HP AT and other passives but not the ascension skills

I'm currently Level 37, and I cannot level up my skills. I've hand hundreds of books none work. The skills all only show level 1 and progression 0%. I've tried everything. My skill training is at 51. Yet my skills themselves aren't leveling up. I've had the same issue yet no way to fix it.