Hi, bad news,probably the curtains are stuck, it happens when there´s too much moisture.
There are many parts on the M´s that are made of steel, they can rust and produce the curtains to stuck or to travel slowly...
For that you may dismantle the entire camera in oder to get into the shafts that hold the curtains.
If you can´t do it send it to a service, it´s not so difficult but for sure body and top cover must be removed.
There´s a plate in the bottom, it´s secured by three screws, once removed you can access the bottom part of the shafts, you may lubricate them, but the top ones cannot be service from that part...
Bye!