With the large bags make sure the bag material doesn't cover the intake port inside the foam seals (center) or it will interfere with the draw from within the bag itself.
Keep the bag level in front of the sealer so the air will rise up through the foam seals. Sometimes pre-shaping the food within the bag, especially with pulled pork, will help put the air into the top of the bag.
I use the 11" X 16" and the 15" X 18" all the time with no problems when sealing whole roasts, hams, and spatched chickens. Let the machine do the work, don't force the cover down after it starts to draw the air out.
