There are many different ways to pin a diaper...here's one for you to try. We may take diaper pinning for granted, but there are many ways to perform this simple task. One way that increases comfort, and really helps the diaper stay in place, is the use of four pins to secure the waist and leg openings.
Tug the diaper to stretch it, then fit it to your body, pinning the waist as snugly as you can. Try to pin the legs toward the outside of the thighs, also as tightly as you can manage. This will avoid the discomfort a pin can cause at the juncture of groin and thigh. Once you have the legs securely pinned, gentle tugging at the diaper should allow any slack to develop at the waist which you can now re-pin and adjust for your comfort.
Using this method, the diaper will remain more securely fastened in all situations than with just one pin on either side.
Remember - cotton stretches when wet, and shrinks when it dries, so to get a snug fit that will stay put takes a bit of practice.