Andrew R Stewart 

What size CO2 cartridge are you using and what tire size are you filling with it. 80PSI might well be enough if you are running tires with an appropriate volume. But I wonder if you're not getting the entire CO2 in the tire. Have you tried a second cartridge to further top off the pressures? Remember that CO2 will bleed out of a tire much quicker that atmospheric air does. So an after the ride is over pressure check might reflect some bleed off

Pump/inflator head gromets do wear over time, as do pump pistons (be they leather or "O" rings) and most will see improvement with periodic cylinder lubing with a thick lube. The check valve, in most floor pump bases, can become clogged with crap and inhibit pressured air flow. Hose connections to the pump or to the chuck can leak. Back in the day all these were serviceable (and liked periodic servicing) but with today's pump designs and the lack of fixing stuff as a prime design goal most pumps are considered disposable by most riders once the pump become sup par.

To repeat Gabe's comment- what are the factors that lead you to "need" 125psi? Andy