Matt, I think the previous posters are on to something. What I suspect is that, because the airline to those 2 are below (further below ?) the water, the air you're forcing into the line is seeking it's 'path of least resistance' by only going through the top 4 jets. Without having to resort to the shopvac, you can try to plug the working lines (you may have to do one or two or perhaps all before the air forces the water out of the other two) - once the air has created a path to the last 2 lines, they should continue to bubble after unplugging the others. I'm speaking from lots of experience, though mine is from plugging multiple lines while closing a few hundred pools each fall. Please give this a try, if it doesn't work, try the shop-vac thing from above. (I realize that this is somewhat of a PITA, but it's just one of your spa's idiosynchrities and once you get used to having to plug the other lines before the other two kick on, you won't think twice about it) (BTW- you could probably just cap them with your palm).