A fitted sheet that slips from a corner during a night change is usually signalling a sizing problem, not bad luck.
Cot and cot bed mattresses are made in several length, width and thickness combinations, so the mattress should be measured before bedding is ordered. A correctly fitted sheet lies flat across the surface and stays secured around all four corners.
It should not be stretched so tightly that the fabric pulls upward, nor should there be spare material that can gather beneath the mattress. Start by measuring the mattress itself, rather than relying on the cot’s advertised name. When comparing cot bed sheets, match the listed mattress dimensions with those measurements.