That could be a bit of a problem. The issue with aep's is that because of the size the internals are very small. This is one reason why fps is so low.
First thing to check is if the pin on the rear of the selector is broken. Second is the housing for the pin, the bit it goes into when the "slide" is on.
If these are in tact then it's an internal issue. I would only recommend opening internal of gun if you have worked on larger aeg's before. Ifs so you could try fitting a smal MOSFET. Very difficult but worth it.