I noticed that your sewing machine is sitting on top of a table instead on down inside. Try and make yourself and ext. table that will go around your machine to assist in holding the weight of the quilt that you are working on. All you have to do to accomplish this is to buy like insulation foam board and then cut out an area for your sewing machine to snuggle into. You can get this in different thickness so just measure your machine and then get the correct thickness. Really a cheap solution.
As someone else stated also, I don't pin. I spray baste and then if a very large quilt I'll hand baste it. Don't want that puppy to move. LOL Makes life much easier. I did a 104x110 this way and NO puckers! WOOOOO HOOOO Also, get something next to you to hold the weight of the quilt that'll help alot.
Good Luck!