Battery not charging when on

depending on which brand you have, this seems like either:

the tablet manufacturers didn't design the hardware to process both signals using power banks separated through relays and switch components. Or the circuit was capable of such features but couldn't sustain the somewhat precision voltage regulation with time.

another equally likely (or tandem) issue lies with your OS. check firmware has a great deal in power management (through digital signals) and can a lot of times hide under the appearance of faulty hardware.

What is the model and brand you have and how long have you had it? one thing you could try is to open up the body and just check if your internal battery is spatially out of place (even the slightest can cause whatever bad luck one has), has a loosen connection at the receptacles (e.g. at the external power input site), general swelling or mishappen body (in which case you gotta throw this battery away and replace asap, etc.