Sounds like the case itself may be warped.
It’s going to be hard to fix. You may be able to sand down the area around the fan. Be sure to use fine grit paper, go slowly, and clean off any loose plastic bits afterward.
I’m guessing you already verified that everything’s aligned properly and attached snugly.










Actually, yes, listen to this guy. Try what he says before you do what I said.
Additive repairs are safer than destructive repairs.