sometimes after charging a battery if it sits for a few hours before you plug it into the truck the voltage may drift down below the reset point the battery guage (BDI) is set for and it will not reset when you plug it in.
You can try charging it for 15 mins and then plug the battery right in and see if it resets then.
If it does then you will have to adjust the reset voltage setting to compensate for this.
If this does not affect the problem and the guage will not reset even after adjusting then the guage may be defective.

Depending on which guage it has the adjustments may be on the back of the guage, if it is an older curtis guage it may have an addition controller it hooks to that has 2 black knobs on it that can adjust the VPC and reset voltages.

you can look on the guage and see which curtis guage it is and go to curtis instruments website and look it up, they have manuals on all their BDI systems there.