Help with Crosman Nitro Venum

I took it apart because it was starting to shoot extremely low quite frequently. Something wrong, but what? I took it apart but can't see (tell) if anything is wrong. Can the piston go bad after three years? It has been a fine shooter until recently. ( A similar thing happened to my Walther Terrus and when I sent it in for warranttee work they said it was not repairable and sent me a new rifle. Maybe this is not fixable either.) Can I troubleshoot parts? I am not seeing the piston seal and the "piston" on the parts list looks quite different from the one in my barrel. Sorry, I am a newb at airgun repair.
Anyway I have it apart and want to be sure to get it back together properly. Working better would be a bonus. Also need an exploded parts view for the trigger if you can point me to one.
Help please.