Use the left index finger to guide the fabric on the left side. Tickle the fabric through the dogs. No need to hold fabric at the back of the machine. It's already gone through and stitched.
Example
https://video.search.yahoo.com/yhs/s...09&action=view
Slow the machine down if you can.
What foot are you using to stitch? Check the different as one may be wider than another. The wider one may help hold down the fabric more. I didn't realize the difference in feet until I took a class about them at the LQS>
Hope that helps!