My charger will turn my phone on, but once its on it does not charge. What does this mean

So, it seems to charge when my phone is off because it turns on after awhile. But once it has turned on, it doesn't seem to be charging. The charger i was using before was the only charger that seemed to work with my phone while it was on or off. Now, Im left with this charger that seems to work but not fully. It charges while its on, on my friends phone but not mine. What do i do?