Daniel became suspicous that MC2 M3/M3 may not be actuating.  I drove a 0.1Hz line on M1 on MC2, and see it clearly in the M3 L witness (osems).  

When driving MC2 M2 with almost the full DAC range (300,000 counts on the test L filter bank), I see no response on M3 witness osems.  Repeating this test on PRM the M3 osems see about a 10 um amplitude motion, so it seems that this level of drive should be visible if the suspension was moving. 

Edited to add:  Marc and Oli found some loose cables, now the actuator on MC2 M2 works.