Hirose-san,

I checked the relations of Y arm transmission power and REFL power.
Figure 1 shows the time series data of TRY NORM and REFL PD output.
Left T cursor shows the timing when TRY NORM is about 1 while right T cursor shows the timing when TRY NORM is about 0.8.
As you can see, REFL power at left T cursor is about 0.23 while right is 0.017.

Since the single arm lock is managed so as locking stably only when REFL power is around 0.23, it might cause problems.
So, please check IFO REFL power if you feel Y arm lock is strange.