gun wont stay cocked

Assuming that it worked before you took it apart, and that you just cleaned it and such, I'd guess that something didn't get put together properly when you reassembled it. First thing to do is to take it apart again, make sure that all the pieces and parts are where they are supposed to go, and make sure it is reassembled properly, and all the screws and fasteners are tight. If that doesn't work, let us know, and also tell us what kind of gun ya got.

Is it definatly a washer, or somthing thar looks like one? Im thinking its your rear buffer o-ring. Not having that may cause similar problems. Also check your ball latch. Carefully match it to your manual.