same here, that´s why i don´t like the DOD FX-25B ...
seems like many people complaining the volume drop, so i believe it´s a "normal" thing. on the other hand i found this information but not sure if the bad pots only refer to the FX25 (without B): It is prone to a specific failure over time where the volume drops by about half. The culprit is the "sensitivity" potentiometer, so just replace it with a new 500K linear pot. As far as I can guess, DOD must have bought a large shipment of bad-quality 500K pots.
Source: ovnilab.com
Search google for "DOD FX-25B mod" ... some people switched one or two restistors but can´t tell you if the results solved the problem really.