stevie boy

having got some some new crstal side lights for my classic, i'm having a bit of trouble fitting them or at least them staying fitted. they seem to sit slightly low at the rear of the light but they are a tight firm fit. until the following day when they have come out of the locating lugs and are all floppy due to just been held in by the screew. anybody else had the same or know what i'm doing wrong?