Do you mean you are using a zig zag foot when you drop the feed dogs? If so, this may be your problem. You need to use a hopping foot, darning foot, or floating foot to FMQ. A zig zag foot requires the feed dogs to be up and does not allow for FMQ.
Edit: Once you have the correct foot, you can experiment with dropping the feed dogs or leaving them up. On some machines it is easier to control the quilt with the feed dogs up. I FMQ on my machine with the feed dogs up, but I am using a hopping foot (darning foot).