why is my NextBook 8 having charge issues?

So I am having issues with 2 Nextbook 8's. The problem is whenever you plug the power cord into the tablet it wont charge so you have to push it in hard and hold it in order to get the red charging light to come on. If I dont hold it in then it will just shut off. What is more likely to be the problem, charging jack or the power cord? any advice would be good, thanks