I had a similar problem after changing the headliner on '87 740. This wind deflector gets a little bent or sprung in the center. The fix is to bend it in the reverse direction - down in the center. Don't be brutal with it, you only need to persuade the center down. You do this by pushing down in the center and gently pulling up on the sides at the same time. This is light weight metal and it will yield fairly easily.

Note: There are two very light weight struts that slide in the tracks on each side of the deflector. You don't need to mess with them ,but be very gentle as you have to start taking things apart if they get broken.

This job takes a whole lot more finesse than force.

Dan