The sunroof on my 2002 Avalanche 2500 just developed a glitch... on 2002 Chevrolet Avalanche 2500

I poened it (slide back style) and now it will not fully close again -- reaches about half an inch from closed and stops, won't pop back up to flush with the roof. I don't see anything blocking, and have heard this isn't uncommon for GM trucks in this era.

Can someone offer diagnosis / correction advice please?