Enchanting level does not go up, no matter how many bookshelves I place!

I was playing in my vanilla survival world, grinding sugar cane and leather so I could make some bookshelves for my newly acquired enchantment table. I reached level 30, and decided to enchant diamond boots. Before I even opened the enchant table, I placed down all of my bookshelves, it was more than enough to get to level 30 enchant, I opened the table, hoping to get depth-strider or feather-falling or something like that. But when I opened the enchantment table, the highest level available to me was lvl 12!
-Max