One of my friends came over to show me his MF28 back on his F5 recently.

I got my MF18 back for my F3 out and we started to compare.

With the fiddling around etc, his MF28 back started to malfunction as you describe.

Eventually it appeared that the battery in the back itself was flat and it was operating from the camera battery(s?).

He changed the MF28 battery and it worked again.

Not sure if this helps, but we found that particular piece of information in the instruction manual.

You know what they say, "if all else fails, read the instruction manual".

This could be your problem.