This is an old thread, but hopefully it will help someone. The issue is that the tablet requires some battery life in order to enter charge mode. With most tablets this is typically around the 4% battery life mark. If you go below this point, the tablet does not have enough power to enter the charge state. This was a common problem for the Black Berry Playbook.
The solution is to use a rapid charger to boost it. The rapid charger that comes with the galaxy s6 or s7 will work. You can also try an external battery pack as they generally run on the same rapid charge principle.