I've had mine out a few times this summer working on an odo reset issue. The pictures are from my spare '72-'74 but the mechanisms are basically the same. Here's what I found. The tension on the vertical spring is critical - too tight and it's hard to reset to zero but all the numbers increment - too loose and only the "1/10th", "ones" & "tens" advance. That spring pushes the plastic pivoting piece forward which is a must for it to work properly. If that piece is too far rearwards then not even the 1/10th will advance. My guess is your spring might be broken or missing.
The reset knobs are readily available from several vendors.