The common error signal on the TTFSS has a 5 mV offset, which was causing the loop to catch on the edge of the error signal, near the sideband. I've adjusted the offset pot on the TTFSS interface board from 502 to 960 to remove this offset, and the loop now catches only on the carrier.
Also, I've taken the DC path from the south cavity RFPD and plugged it into an SR560 with gain 10 and then into C3:PSL-RCAV_FMON. This is temporary, and I've done it so that I can remotely lock the south cavity more easily for the gyro beat measurement. With the gain of the SR560, refl on resonance is about 2 V at minimum.