I do not know when and what it starts but it seems I have about 10 more frequencies(CPM) for the same shaft.

for example, #7 iron 37 inches with grip, NSpro950R, I used to have 285 but now I have about 295-299.

I have been using the machine for 4 years and it was working fine.

let me know if there is re-calibration method.  I am not able to send it to you as I am in oversea.

or, it is something to related to hold down clamp?

Britt Lindsey Answered question January 12, 2023

Couple of things:
shaft frequency readings could change if you have a different grip on then you did previously. It is not advisable to measure frequency with a grip on as it does not give a true reading of the shaft frequency. Different grips and aged grips have different density’s and can alter the cpm reading. Gripped clubs will generally give a lower cpm reading than ungripped gloves.

your hold down torque knob could be out of adjustment which can occur over time. No direct procedure with the instructions for this model, but I believe the torque knob is the same or similar to the the one used on the Auditor. See this link:
around page 9 there is a procedure to adjust the torque knob.

Hopefully you made a reference shaft when you first received the unit.
Note: my auditor measures the same shaft at 283 cpm @36.5″ with no grip.