It has happened to everyone. You pull the fitted sheet over one corner, and when you reach for the opposite corner, the first one springs off. The problem is usually not your skill but the size.
The measurement most often overlooked with fitted sheets is depth. Your mattress may be 160 × 200 cm, but is it 20 cm deep or 30 cm? If you have added a soft topper as well, the number grows further. Measuring the mattress with a tape before buying and making sure the sheet's pocket depth is at least 5 cm more than that puts an end to years of small irritation.
Then there is shrinkage. A cotton fabric can shrink a little in the first washes. Good manufacturers allow for this or pre-shrink the fabric. Duvet covers raise a similar question of fit. If your duvet is 195 × 215 cm, a cover of around 200 × 220 cm lets the duvet sit snugly in the corners without getting lost inside. Those few centimetres change how the bed looks and how much effort it takes every morning.