Most likely one of the rollers is broken on the cranking arm inside the door. You will have to take the door panel off to see what exactly is wrong. If you haven't done this before I recomend you purchase a Chiltons manual for your year camaro.
another tip. when / If you replace the roller wheels, take the extra time to replace the felt on the tracking, and then shoot it with some lithium grease. (in the tracking) this will keep it rolling smooth for years.