I can only fix what I know works. Take, for example, one of these handy devices. If mine breaks in half, I can transmute so that it looks like it's repaired, but I don't know how the innards function. Ergo, it won't work.

My automail is much more complicated than a 'peg leg.' The wires connect to my nerves and while I can make very basic repairs in an emergency, I don't know how all the nerves and wires and innards work in order to repair it on my own.

-=A small huff=- Besides, I don't want to put my mechanic out of business!