Mattresses have an enormous variance in thickness, and sheets vary considerably in the depth of the "pocket." If your sheets can barely fit over a too-thick mattress, then they're going to pop off when you move on them. You might just want to buy some new sheets with a deeper pocket. Alternatively, you can buy elastic straps that go under the bed and clip onto the sheets. You'll generally either find these as a four pack of short ones that are meant to go only at the corners, or as a two pack of long ones meant to stretch under the entire bed and attach opposite sides. Get the long ones; they generally perform much better.