Following the suggestions from Rob and Bogdan, I've increased the HV setpoints
of s0 by 200V in each PMT.
Here are those new setpoints:
Left Arm HV
Card 2, Ch 10 = -1400
Card 2, Ch 11 = -1550
(card 2 labelled "Gas Cerenkov")
Right Arm Top HV
Card 14, Ch 10 = -1550
Card 14, Ch 11 = -1550
(card 14 labelled "Gas Cerenkov")