Hello,
around and between the bayonet flange and the mirror box on the camera body there is a moving ring which is spring loaded. A little step couples with a nose on an Ai or AiS lens and transfers the f-stop set on the lens to the meter system, the ring follows the movement of the f-stop ring. If an old non Ai lens (not converted to Ai) shall be used, on some models like the F3 or F 2 AS, this little step on the body has to be flipped away and then you have stop down measuring. I think the FE, FM have this flip-away step, I'm not sure if the newer FM 2n or FE 2 have this device any longer. If you mount an old non Ai or AiS lens (not converted to Ai) to such a camera with force maybe you could bent or damage the coupling. Check whether the spring of the ring is o.K. or broken and have a look into the instruction manual for use of non Ai lenses. If the coupling is damaged, the repair could get expensive as the camera has to be dismantled nearly completely (top cover, flange ...).