I had this problem, too, and one issue was that the plastic sheet on the top was warped; so even when I managed to slide the silicone dome into place the first time, it popped out. I’m replacing my computer soon (not under apple care any longer/will cost hundreds to repair), so I just went ham with a toothpick and glue stick, hoping that inserting bits of glue will stabilize the dome for the next two or so weeks. (My key affected is ‘T’, so you can imagine the headache).