This is a bathroom type of exhaust fan? Did you check it when it was first used? A lot of the "less expensive" fans have little or no air movement. I've never heard of one with the blades flattening, it would seem the blades wouldn't flatten at an even rate and would be very noisy-out of balance. Ain't saying it can't be done though, one never knows, where was it made?
is it possible it is wired backwards?
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain