The enchantments offered on the enchantment table do not change, even when other items are enchanted. The only options available are Unbreaking level 1-3. I have applied these enchantments and then removed them using the grindstone multiple times, but I am still only offered unbreaking 1-3. I have broken the enchantment table and rebuilt it, with no avail. 

I am using the bookshelves as recommended in the correct format, and lapis lazuli. My XP level is/was >30. 

I do not have this problem when playing on the PS4, only on iOS.