enchantment table bug

I have enough bookshelves around my enchantment table to get it to level 30, however the highest level is never 30. It's usually either 12, 18, or 20 max. I've tried taking it down and setting up the bookshelves in different ways but nothing seems to work. It also doesn't seem to matter what my character level is, even when i have 30 or more it still shows up as less than 30 on the enchantment table.