I have a Nikon FG that it's shutter only fires on the settings: B and M90. When I put it on 1 thru 1000 & on A or P the only thing that happens is the mirror pops up and stays in the upright position. The shutter doesn't move. If I then move the settings control to M90 or B the shutter will fire. The metering also is not working. Any help would be great! Thanks, Jean.