As I noticed that nether works, I went into it and mined my way through it with my Tinkertool-Pickaxe. As it leveled up, it didnt give me a modifier, it just said "Your Diamond Pickaxe has leveled up to …". I tried it another time to make sure that my thoughts were correct. It didnt get a modifier the second time as well. So i decided to try in overworld, where it worked fine and i got a modifier on my Tinkertool.