Pardon me if I speak from SAAB rather than BMW experience, but these things usually, regardless of brand, have some kind of friction clutch as a safety device to prevent cutting off the occasional hand being held up in the breeze, etc., and they do wear out. My solution for the #2 daughter with the '90 900S was to equip her with a rechargeable electric screwdriver with the proper size bit so that she doesn't have to crank for 5 minutes after the downpour starts on those occasions when the factory motor doesn't do its job.