The battery may be failing. How old is it? Even rechargeable batteries have a life span.
Here is a link for a replacement battery- not a recommendation to purchase from them only to show the price and specs.
Here is a link on how to open a Nextbook 10 (not the same model I know but maybe close enough) so that you can see the battery to replace it if you feel that is what you want to do. Your model only has 1 battery.
The battery discharging while the tablet is connected to a charger could be due to :
The charger not providing correct voltage/current to run the tablet and charge the battery to the correct voltage. Use a DMM (Digital Multimeter) to measure the Voltage out of the charger. It should read 5V DC. Anything less than 4.8V means that it should be replaced. If you do not have access to a DMM try to access another charger. Ensure that its' output rating is 5V 2A DC. The rating will be printed on the charger. Also it could be because the battery is failing to hold its' charge. A good battery should maintain its' level of charge when the tablet is running on a charger with the correct output.
It could also be a faulty charging circuit in the tablet, which would be an option if you have already replaced the battery and the charger and the problem is still evident.
Hopefully this is of some help.