Hmmm if it was wireless charging that would be somewhat normal. But since it is micro USB it could be a number of different things.
It could be a bad micro USB cable. I would try another cable in this scenario to see if it makes a difference. Also check the adapter that allows it to plug into a power outlet. That's another thing are you charging via USB port or power outlet?
Another possible issue could be the micro USB port on the phone. Look to see that it is not damaged in anyway. See if the cable is loose in the port but be careful not to damage the port.
Another possible issue could be the port or power outlet the phone is plugged into. If it is a computer charging the phone it could be a faulty USB port. But if it is a power outlet it could just be dirty inconsistent power. Or maybe there is too many things using the electricity on the breaker the outlet is on.
There are possibly many more scenarios. But I would look at trying another cable first, then check the port, then possibly switch outlets.
Hopefully this helpful information!