Also, you have to make sure you're using the correct standoff and in the correct orientation. The correct ones for AM4 socket has a washer like thing attacked to one end that can't be removed. That end is what screws into the backplate
I am just now installing this same cooler. A sudden idea popped in to my head when I encountered the same issue. If you use the thumb screws, and grip them tightly, you can screw the standoffs all the way down