There's a rubber cover to hold the mushroom in place, which presumably you have lost. That rubber cover is supposed to be super-glued in place. Either it wasn't fixed well enough, or someone has removed it for dismantling and not used superglue when refitting. (It looks to me as if the rubber was designed to just press into place but it didn't work well enough, so the superglue was a last-minute fix.)
Insulating tape should do the job temporarily until you get a replacement rubber cover.