You should invest in this book:
http://www.amazon.com/gp/product/160...JFFWRH7PZWMJ8H
I had the same problem and took off the needle plate to clean it out. Each time I thought I had it back together, it wouldn't form a stitch. Never thought I'd need a book for such a task, but I would have never gotten it back together without the book. It already saved me a bundle because I would have had to take it somewhere to be fixed.