Camera winds but shutter doesn't fire?

Hi. I am having a problem with one of my F4s'. Sometimes when you press the shutter button the shutter and mirror

do not move but I can hear the motor attempting to avance the film. This just started a few days ago. Luckily, I

had a polaroid back attached to this camera so no film has been wasted. Does anyone know what could be causing

this? I'd say it does this almost 50% of the time.

I posted a response to a similar problem awhile back. The person that started the thread didn't post a reply as

to whether my suggestion was correct or not.

 

http://www.photo.net/nikon-camera-forum/00PBD4

 

Basically, my guess was that the camera thinks there is a new roll inside and tries to run it up to frame one.

The little lever in this photo seems to be what tells the camera that the back has been opened. If the

back is

opened, or pressure is relaxed at this point, the camera thinks there is a new roll.<div></div>

I considered that too but the camera used to fire even when the film door was open. If that lever is the problem it would explain why this has only recently been noticed because the polaroid back was never on this camera before.

Does it work properly with the normal film back? If it does the problem is the Polaroid back or how it mates to that particular camera. If it does not work properly with the normal back, then the problem is with the camera.

It appears to work fine with the normal back on it. I am going to play with it for a few days (without film loaded) to see if the problem persists.