I am having this same issue which has been discussed on these threads.  I took the steps outlined by someone from Dell in this particular thread:

http://en.community.dell.com/support-forums/laptop/f/3518/p/19479123/20242810.aspx#20242810

Still am having issues.  I'm having to keep plugging and unplugging and plugging in the laptop over and over until it begins charging.  I spent 30 minutes tonight doing this until the laptop finally started charging.

Is there a fix for this?  This is definitely an issue that is known to Dell.