Conkeldurr

I'm talking about my Aggron, although it's happened with toxicroak too I made a Rockhead Aron because I wanted a headsmash set. But after he evolved his ability changed to sturdy same thing with crogunk and dry skin. His changed to anticipation

hi. I know this has probably been answered a billion times, but I'm new to pokegening so bare with me please I have a problem with my created pokemon abilities changing after they evolve. Like if a pokemon has 2 abilities and I choose the one I want, after it evolves it changes to the crappy one. It doesn't happen to all of them either, just a few. is there something I'm doing wrong in the pokegen process? I always make them into eggs o that I don't have to bother with the trainer ID and I always use the PID search function to make sure their valid thanks