One thing that can affect boost is the connection between converter and battery.. Now actually on the PD's Boost and absorption are basically the same (Boost is current limited, Absorption voltage limited but as I recall (Can look up) same voltage) so your Tri-metric may not show the BOOST mode properly.
ALso, line resistance (Wire and connections between converter and battery) may limit your current and knock any converter out of "Boost".
From the manual (PD 9105)
Lignt on steady 14.4 volts
Light blinks once or more per second 13.6 Volts
Light blinks every 6-8 Seconds 13.2 volts
They call it Boost, Normal and Storage.
