Pro-Guide AGM only reaches 75% Charge

First, do a lead swap of the charger banks to see if the problem follows the battery or the charger.  That way you can at least determine whether the issue is battery or charger.

An AGM should read about 12.8 volts after surface charge has dissipated, which is about 8-10 hours after removing from the charger.  If it is the battery, as BCB suggested, you can also try to charge it manually as it is possible that it was run down below the threshold where the charger recognizes the battery (although not likely in your case as usually no LEDS illuminate in that scenario).

Also assuming that there is no parasitic drain from accessories while charging.

The reason that Don Weed suggests at least 950 ccas is because the Opti alone requires 800 ccas just to crank it.  He suggests exceeding that by a minimum of 150 ccas to allow for accessories like livewells, graphs, shallow anchors, etc,. in a typical bass boat. That X2 made by Northstar is a real beast, and probably the best 31 AGM around at the moment, along with the Odyssey.

My 3 year old pro guide was limping along as well. Would work fine but tested faulty. I also went the same route as swimrunbike, and got a x2 from batteries plus. Well worth the investment just for the higher cca ratings alone.