I was having the same problem - so what I did was to take a piece of paper and with no thread in the machine try sewing in under the guide - seam was too wide; then i went right up against the guide - it was still a smidge too wide. FINALLY I went to the spot where I could just see a sliver of daylight between the quide and the fabric - VIOLA!
The other thing that is really important (and I have to credit either the Better Homes and Gardens Complete Guide to Quilting or Harriet Hargraves - Freshman Year Quilt book) - watch the FABRIC. The feed dogs and the needle / thread will do their job - your job is to WATCH THAT FABRIC and make sure it goes where it is supposed to!
good luck!