I bought a used FM off ebay a fortnight back - cheap - because the 'meter not responding'. I have used it with a handheld meter, and the first two films through have come out perfect, using three different lenses. Apart from the meter not responding correctly, the camera is in exc+++ condition. I might even go for another FM.
Re the meter, I have been told it may be dust, and to rotate the speed dial, and the ISO dial, and that may help. I am now getting reaction from the meter on occasions, - both + & - readings, besides correct exposure indication.
Other suggestions are to clean the battery terminals.
One guy posted diagrams as to dismantling the top and showed the inside. NOT for me. If the meter does not reactivate eventually, then I can still use it with a handheld meter.