i bought four emerson tablet model 744 for my children. None of them seem to hold a charge one of them i cant get to charge. The one that i can not get to charge won't even turn on when when i have held in on button for 30 sec. i have hit the reset i check the charger with one of the other ones. so it not the charger. I am running out of option pls help. I can not remove the back to take the batterie out. Any suggestion to fix this problem would be greatly appreciated.