What size pins are you using
When you pin baste do you close each pin when inserted or do you leave them until an area is pinned.
If you close them when inserting into the layers there is a tendency for the back to pucker a bit.
Also if you base on a table (and yes you can baste a large quilt on a small table) then stretch the backing and clamp to the edge of the table to keep it tight. Do the same when you reposition. Also, as stated above, don't close pins until you have an area pinned and then close