Pack and Play Mattress Buckles

Sheet is too small. Use a different one. A twin size can be folded almost in half and tucked under enough to hold it on when the baby is in it.

I just put down and big, thick blanket to weight down the mattress then put the sheet over top the blanket and mattress. It makes a tighter fit and weighs it down. And with the sheet still being on top there isn't a blanket that can cover the baby's face or anything.

Try a larger sheet or just push down on it, if it still buckles, then def. a larger sheet.

Our pack n play has velcro straps that go from the mattress pad down through some slits in the base and attach underneath to keep the mattress from buckling ... maybe yours has this? (I didn't find them right away either and had the same problem :-D )

you might be able to stretch the sheet back out while wet from the washer. then hang dry. i am assuming it shrunk a little in the dryer. you might find a larger one and see if it shrinks.

i never used a sheet on our pack n play. our son used it for a couple hours a day (in intervals) we used a blanket under him or nothing at all if he was playing in it.