Retina frame counter

Hello
I have just finished test roll with my new Retina IIIc. Photos are good, camera works well with one exception. Frame counter stopped working arund 20 number on a dial. After removing film I was able to reset counter, without film it seems to work OK. But might stop again with new film. I have found some good CLA tutorials on the web, but none of them actually showing frame counter mechanism itself. Do you think it maybe only problem with dirt? With photos and some tips I may try to clean it.
M_alice
PS. What fantastic lens in that camera.

You probably know this, but just in case...

The frame counter counts down, not up.

From the manual:

"Remember to set the film counter
If you are using a 20-exposure cassette, set to the index
mark between No. 20 and 25. Now work the rapidwinding
lever and press the film release button (8). Repeat
this until the film counter indicates No. 36 or 20
respectively. At the same time the rewind knob (20)
should rotate against its arrow if the film is advancing
properly. The film counter always indicates the number of
shots still available. When it has reached No. 1. - in other
words when the whole film is exposed - a transport lo*
comes into operation. Neither the rapid-winding lever nor
the release button will work further."