The first rag quilt I made I washed at home in my top load, adgetating washer and it frayed really well.
I made my second rag quilt and had the same problem you are having. It wouldn't fray. The only difference is that I took it to the laundry mat and put it in a front load washer. I didn't have time to wash it again at home.
It was the same pattern so that couldn't have been an issue. They were 1/2" seam allowance and I cut 1/2" apart on both quilts.
I can only assume the front load washer was not enough motion to cause fraying.
Good luck.
Christine