Have you lowered the presser foot? That is a common problem with machine quilters. The quilt sandwich is thick and it looks like it's lowered and it isn't. Also are you moving the quilt itself? You are in control of the stitching on the quilt, which ever way you move it, but you have to move the quilt or you will be just stitching in one spot since the feed dogs are down.
There isn't really a difference in a darning foot and the free motion foot except the name. If you continue to have problems I'd suggest you take your machine back to the dealer and ask for some instruction in doing the free motion and also have them make sure that the machine isn't the problem and not what you are doing.
Hang in there, girl, as free motion quilting isn't learned in a day or two, it requires practice, practice, and more practice! :) Have a practice sandwich of something you can pull out every day or so and do free motion on, also maybe buy a book on machine quilting. I recommend Harriet Hargrave's Heirloom Machine Quilting or one by Diane Gaydusack, I think thats how to spell her name.
Good luck.