Hi and welcome!
I have a Gus Kiss that has a fuse in the top, it looks like your mod may have one too. I can't tell from the pictures on the Gus site. Does the inside of the top look like this?
EEK! Sorry for the size of the picture. Anyway, the silver square thingy is adjustable. The one on mine is fussy and some attachments squish it down, then the next one I put on won't fire. Try taking the top apart and looking at it closely. It may need to be raised up or pushed down. To move very the top up lift it with your fingernail or a flathead screwdriver. Very gently! It's part of the fuse and not supposed to come completely off. If it seems to high, just press it down.
I like my Gus, but I've found that it doesn't like some of my attachments. That little square can be a problem.