Just as a guess, you're plugging it in then, maybe in 15 minutes, you're checking to see if it has any charge. The first 4-5 hours of "charging" a dead battery doesn't put much charge into it, it just ramps up to the charge point. Leave the charger plugged in (with the tablet turned off) for 24 hours. Then see if it charged.
No sense chasing down a problem if there's no problem.