P90 stops, shorting makes it work again

[KenIchikawa]

One of my P90 works fine. It then sometimes stops.

The way to get it working again is to pull off the fake ejection port cover at the bottom and shorting the trigger contacts together.

It sparks a little and behaves normally.

What's going on here? What can I replace/fix to stop the need to bring pliers to take off the cover to short the trigger contacts?

Thanks