Is your sandwich well basted with thread or lots of safety pins? I like 505 spray or Hobbs 80/20 fusible batt to keep the sandwich from getting puckers on the back.
I find it best to stabilize the sandwich with lines of machine stitching in the sashing if possible. I would use my walking foot to stitch down the center sashing line first. I would then stitch the horizontal center line next. I would then stitch the top horizontal line next. After these lines are stitched I would switch to my FMQ foot and start outline quilting the motifs. After they are outline stitched I would use a background FMQ filler like meander.