Rana noticed that the sum on WFS2 was about 10 times smaller than that on WFS1. Though the beam appeared centered on the DC QPD screens it was not really true. When I went and checked the actual beam position it was landing on the metal enclosure of the WFS2 sensor and scattering back on to the diode.
I also checked the power levels of light landing on the sensors It was about 0.25mW in both cases. This needs further investigation since the power split at the beam spitter is like 0.25mW onto WFS1 and 0.45 towards WFS2. The lost 0,20 mW has to be traced and we have to be sure that it is not scattered around on the table.