I had the same issue when quilting a baby blanket. I started quilting it with a standard needle and realized I forgot to switch to a quilting needle about half way through my first echoed block. I finish the block and then switched needles. Then the trouble happened. My machine would not quilt. I put a piece of practice sandwich together and tried adjusting the stitch length and it would still skip stitches. After a huge mental breakdown, (because it was two weeks until Christmas and I still had gifts to finish), my husband suggested I google it! With two seconds I found a list of reasons why a sewing machine skips stitches. The number one reason is the needle! Try changing your needle, it worked for me! Good luck and Merry Christmas! Here is the link if it is not the needle,
http://sewing.about.com/od/sewingmac...roubleshot.htm