It could be a few things.
1. If you are charging it while it is on (not off) something on the device could be draining the battery at the same time. Making it take longer than normal to charge. To resolve that you would either have to charge it when it is turned off, or figure out which app(s) are the issue and make sure they are turned off when you aren't using them (not just out of sight, actually turned off) or uninstall them.
2. Your charger cord could be only intermittently charging the tablet. If that is the case you could try another charger cord.
3. Your battery is going and needs replacing.