Blackberry

I have been following Jenny Doan's way of putting on binding. I use 2 1/2 inch binding, fold in half, press and attach to quilt. When joining ends I measure 2 1/2 inches and cut the top strip, then make a diagonal seam and press the seam open and the binding down on the quilt. At this point everything is fine but as I am attaching the binding the first thing I know is there is a small wrinkle on the top half of the binding and the further I sew the bigger it gets so at the end I have a nice little pucker. What could be the problem since it lays so nice before I begin to sew.