The short version is that it might need to be serviced. It could be the shutter, especially if it's a leaf shutter. It could also be a worn part.

The quick fix is to put a bit of lighter fluid (small amount - short squirt) into the side of the shutter. Look for an open slot. If this seems to correct it, then the shutter might need to be serviced.

It can hard to diagnose this kind of problem without testing the camera.