We have a whole slew of Jandd panniers here. 3 full sets of 4 panniers each. We use them a lot. They last forever.

Do yours have a plastic piece of hardware that the strap runs through to tighten it? If you lift the plastic piece it should loosen the strap.

Ok I found a picture:



It's the tension lock that I am referring to.
The strap that goes through it should have velcro on the end to hold it down. You'll have to undo the velcro to lift the tension lock piece.