RYOBI Drill Driver chuck hard to adjust

The chuck might have debris in the jaws, which could make adjusting the chuck harder or stiff.

Open the chuck fully and clean out any build-up of dirt and dust - preferably with compressed air. 

This can be done by blowing compressed air (either from a can of compressed air or an air compressor) into the open chuck around the internal jaws until build up is removed. 

You can also use a small pipe cleaner (cylindrical brush) to clean the debris. 

You may need to cycle open and closed the chuck jaws to loosen all the debris.