Euro tail light popping out

the drivers side light stays in fine and firm.  the passenger light is loose and pops out sometimes.  i see when it's popped out that the right side of the light is held in by putting the light in and turning it, causing the metal piece sticking out to be held under the fiberglass.  the left side doesn't have this though.  the left side just has a bent metal piece, where when you put it in you would bend the metal piece so that it fits in, then when it is in the metal piece would bend back out to sort of hold itself in.  But it doesn't hold very strong.  Am i missing something here?  i don't wanna take the driver side one out to see why it stays in so good, in case that messes it up.