We have not been having any issue with them and we sell hundred of them a month.
Are your customer properly disengaging the chain brake before trying to take off the cover?
If they do not pull the chain break back before taking off the cover, the cover will jam in place. If they unscrew or play with the cover before unlocking the chain break, this too could cause the cover to jam in place and then it would have to be taking off by the service tech.