Can"t enchant more than 20 levels.

When I go to my Enchantment table to enchant something, whether it be a sword or a bow or a book, anything, it will not let me have an enchant above 20 levels. This was not originally an issue, as the first enchant I did on a diamond sword used 30 XP. Now, when I go to enchant something with 30 levels, I do have at least 31 XP and above. I clearly have enough and yet it will only list around 5, around 10, and 20. I also definitely have enough bookshelves around my Enchantment Table. I have more than 15. This has been very frustrating as I would love to try to get Fire Aspect I or II and Fortune I or II on my diamond sword and pick. I have tried destroying the Enchantment Table and bookshelves and replacing them, but it still won't let me enchant with more than 20 xp. I think it's just a problem for me, but I don't know how to fix it. I'm also not using any mods either, just good old vanilla. Help me please!