Why is my power brick not working?

A recent issue has made my 2012 Dell Inspiron 14z almost unusable. Without any warning or error message, my power brick no longer charges the battery. More specifically, I plug in the power cable to the wall, see the green indicator light on the brick, plug the adapter into my laptop and for a split second, the screen indicator for battery switches to charging, but then it switches back to battery mode and the green indicator light on the brick goes off. I can charge when the laptop is off but not when it's on. Please help! I ordered a brand new adapter and it did the same thing!