as vintage says........the plastic wasn't heated enough, but also the former wasn't left to suck out the air for long enough...........either first or last of a batch where the operator was just dog tired and fed up.......I used to vac form simple shapes for the kids in school, and at the end of the day when you had 30 plus sheets to do for prep for next day you'd try to scimp on the heating up process, and they would come out exactly like your piece.........it would take you twice as long as if you'd waited those extra seconds for the plastic to be like a hot springy latex membrane before turning the pump on and pulling the leaver down to bring the tooling up under the plastic to make contact ............but then sometimes your eye would be taken off the job..........leave it too long and a nice hole would appear in the middle of the clamped sheet.........but get the piece just right and you'd get the perfect vac formed piece..........ahhh satisfaction!!!..