Its normally the last to be born that suffers that fate as by that stage its like shelling peas so they come out so fast it doesn't catch on anything. To add to that the ewe is preoccupied so doesn't get there quickly enough. Are you lambing where you normally do? If indoors, straw normally breaks it on impact as does a bit of grass coverage/slightly rough ground if your outside.
It might also be the case they have been dead before they came out which could be due to a number of reasons.