It could be that the flap gpt stuck open. When the little plastic pins get gunked up with road grimes they can stick open.
A bit of silicone spray on a regular basis will keep the pins clean and free moving. Don't get any silicone on the back of the flap, because that's where you want to duct tape pennies. It's a trial and error thing you want just enough weight to return the flap and hold it down when you turn the fan off.
When I was a kid I was going nowhere fast, now I'm an old man I'm going everywhere slow.