For free motion, you need a foot that "hops". This is usually a darning foot. You don't actually need to drop the feed dogs for free motion; some quilters find it easier to move the fabric if they leave the feed dogs up.
If you haven't already, take all the thread out and re-thread the machine. Make sure the presser foot is up when you do this; otherwise the thread may not seat correctly in the upper tension discs.
Does the machine make good stitches when you change back from free motion to regular stitching?