Pack n Play Sheets Problem

So we got a pack n play newborn2toddler playard and I was planning on having baby sleep in the infant bassinet at the top. I put the fitted sheet I had got for it on the mattress and it seems to be pulling on it and “buckling” causing it to kind of pop up in the middle. Has anyone dealt with this problem before ? Do I need different sheets? If so any recommendations would be great . Thanks !!