I am probably not getting all the air out of the bag.
Does, for example, a half inch air bubble on top of the bag when you lay it in the freezer make a difference?
I don't see how you can get all the air out of the bag even with submersing the bag in the sink unless you have a sink that is 15 inches or more deep (diaginal dimensions of a freezer bag) and even then seems like it wold take at least three hands to do it. Maybe you just have to get most of the air out?