N64 with XBOX360 controller – no trigger button

I have an RPi2 with Retropie that is working well. I use an XBOX360 wireless controller, but when playing the N64 games the N64’s trigger (Z) does not seem to be mapped to the controller

For example I can’t use powerups on Diddy Kong Racing, but other buttons seem OK.

I’m using the Mupen64Plus emulator for that particular game, whoch plays well

How can I fix the N64 button mapping for all N64 games?

Thanks