You found your problem. Your quilt needs to be supported on all
sides. If it's sitting in your lap it's causing drag just like if it's
falling off the table. I usually put an ironing board on my left
and I have a big table in front of my machine. Try working
from the centre and work your way out. If it's bunched up
on your lap, try to move some so it's free to go under the
needle...sew, move, stop, repeat.
I use Warm and Natural in most of my quilts. For big quilts
I try to quilt in sections because it can get quite heavy.
Something to consider for next time.