power adapter/charger does not stay connected or charge properly

I have gone through three adapters assuming they were the problem for not connecting properly. The first was obvious as the wires became exposed. The second was purchased less than two months ago and it would not stay firmly in the tablet to maintain a connection. The latest and third was purchased little more than a week ago and is having the same problem. Initially it stayed firmly attached now it will not. I am careful with it and am the only one that uses this tablet. What could the problem be?