Right under where the triggers were, you should see some small loose rectangular pieces of circuit board. These are the sensors that register the clicks of the triggers. Lightly blow on them and dust them gently in order to clean them. If they move out of place, simply move them back into place.

If needed, you can also remove the trigger buttons from the case, you can replace these with newer ones if they are too rundown for your liking or if they become damaged.