Had the same problem with mine and I gently levered them from outside with a popsicle stick.
Someone suggested to me to use something called Aerospace Protectant 303 on teh rubber to prevent this - I think it isd available from Jackson centre, but also many marine stores carry it.
I bought some, but have not had chance to try it yet - it is an UV protectant and looks to me a lot like Armorall.
I also added a suction cup with vacuum release to my trailer toolbox so that if the windows should stick again, I will try that to pull on the glass from outside rather than prying the glass and rink breaking it.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.