If the iPad is in operation at this low battery level, it requires more energy than the charger can provide. This further discharges the battery. Switch the device to standby or off, then the battery will charge faster. Bruss Bernhard

With both my smartphone (Android) and my iPad, I always make sure that I don't let the battery get below 30 % and I only charge them both to a maximum of 95 %. So I can't contribute any experience to your problem. My 11-year-old Google smartphone, which I only use as an alarm clock on my bedside table, still runs this way without any battery problems.