My android tablet is not charging while it's switched on but while...

This sounds like it might be a problem with your charging adapter.

1. What are the Voltage and Ampere values for the charger you are currently using?
2. Try using a different charger, and try plugging it into a different socket
3. If this still hasn’t solved your issue, it might be that your battery needs to be recalibrated. Switch the tablet off, charge it overnight, then switch it on again and let it run to 1 or 2 percent. Check how quickly the battery empties, if its unusually fast then maybe you have a faulty battery.

Are you able to physically access the battery? If so, maybe uploading a few photos of the tablet and the battery could help further.