pocket door adjustment

If you are talking about the nut on the roller hardware, that is for locking the bolt in place, check to see if the bolt itself is turning.

if you lift the door a tiny amount with a shim it will help to figure out the tightening/ loosening part of the door without the weight of the door hindering your efforts. there should be a threaded bolt that has a special wrench that goes up or down with the wrench