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.