| In my reckless pursuit of perfect rolled hems an issue came up starting a rolled hem edge. The foward feed dogs would pull the fabric in until the already hemmed edge was between the front and rear feed dogs where there is a gap. The rolled edge lifts the presser foot and the grip on the fabric is lost.
Solution? Cut a piece of stiff paper ~3" by 2". Lift the presser foot and position the long edge of the paper over only the left hand teeth of both dogs, then lower presser foot. Use a pen to trace the edge of the foot onto the paper, now there is a sight line showing how far the paper can go in without being stitched.
Now when you have to feed an already hemmed edge through, position the fabric as usual but with the presser foot up. Carefully lower the foot with the paper lined up with the mark made earlier making sure it is over both dogs.
Now when you serge the paper is caught by both dogs and pulls the fabric far enough that the rear dogs grab and thats it! Dont worry about the paper, it just goes along for the ride but keep it with your notions.
"The geeks shall inherit the work"
p,s, after I wrote this, I found that a piece of htin woven with medium stabilizer gives a more positive grip and feeds even better.