I found that the IMC lost lock more frequently than last week. I tried to find the reason and it seems lost lock when the air conditioner switched on and off.
The attached strip tool picture is the IMC and FSS signals. The upper dark green(yomogi-iro) plot is the transmission. You can see the transmitted power are fluctuating and the IMC seems to lost lock often when the transmitted power is at the peak or bottom. The fluctuation of transmitted light has coherence with the PSL room temperature fluctuation, so the peak or bottom means the timing of air conditioner switching.

I'm not sure what changed situation but one possibility is that we changed input power into IMC many times and current power is different from last week.