I'm trying to add a zipper to a bow tuck purse. I'm following the tutorial listed on this board. However, every time I try to sew the zipper flap onto the purse, the machine goes nuts and I get a birds nest on the bottom. I have tried 3 different machines and every one does the same darn thing. Is it just too bulky or do I need to do something to the machine? The frustrating thing is I added a zipper a month and didn't have this trouble! Any suggestions?