On pickaxes, if you use level 15, there's a higher chance the game will pick fortune 1, because it tries to offer you the most expensive enchantment possible. If it does that, then silk touch can't be applied, since you can't have both silk touch and fortune on the same tool. This is why I recommend using exactly level 14. :)
So your best bet is to pick enchantments in the enchanting table that show level 14 only.
For the highest changes (by far!!) to be shown level 14 in the enchantment table, place exactly 7 books near it :)
Try that out in creative to see if it helps!