Mine were, too. Try SailKote. It has become my go-to product for stuck zippers on the boat.Yes they have zippers but they are jammed. We were thinking maybe trying to was the acutal cushion part to, and hopefully get them smelling better too.
Once I got the covers off, I just washed them on delicate and then air dried them on my deck.
(It's even in the Sail Net store, under "maintainence").