Why my laptop some times showing pluged not charging

I've seen this kind of problem happen with third party adapters. Sometimes they work, and sometimes they don't. From what I understand, it has to do with a chip that communicates with you computer to determine the correct power output. With third party chargers you often get ones that have faulty or unreliable results.

I've seen people go through 5 or 6 of these cheap chargers before they get a good one.

If this is the case, I'd recommend getting one directly from Dell. It's the only way you can be certain that you're getting the genuine article. Plus, they tend to last longer than the other ones in my experience. It's just a whole lot more expensive.

The exact reason is not known indeed. But here it is the possible solutions work for me:

Unplug charger both from PC and from the wall plug. Leave it alone up to 1-2 hours until it is cooled and decharged entirely based some inside processors, capacitors etc. Finally, plug firstly to the wall and then to the PC. When plugging to the PC, you can attemt a little bit hard push. I don't know why but it generally worked for me.