Using too much soap can create a build-up on the material that can take away from its soft texture. The same goes for using too much fabric softener, despite its name.
For best results, opt for a mild detergent that will simply freshen your sheets rather than deep-clean them with harsh chemicals. If possible, you can also spot-treat your sheets rather than completely submerging them on a regular basis.
If you’ve noticed that your fleece sheets aren’t as fluffy as they used to be, there’s also a trick for that. After washing them, let them air dry. Then, comb the material with a soft-bristle brush. This should help restore their original texture.