QE, I have the same problem and figured it out. It is the size difference between the practice piece (which was table runner size) and the actual quilt (quilt size). I had to put another support beside me to support the additional weight of the actual quilt. It was pulling too much until I added the support. I use my ironing board lowered to the right height and cover it with a trash bag so it's slippy and the quilt moves smoothly. I hope this makes sense--it was an "aha moment" and works for me.
DD, that's a lovely block for Bev. Did you ever receive the MB I mailed on June 20? If not, please let me know so I can make you another one. I'd feel bad if you were short a block when it's time to put the MB quilt together.