Well, I know it has been discussed a long time in the past, but rather than resurrect that thread, I thought I'd start something newer as I have a solution too.
As some here have experienced, the Climateline hose for the Resmed S9 can stretch at the mask coupler end. If the hose runs a little funny, it can tug on the mask and stretch the end of the hose. Now, you'd think the rubber / silicone end would return to the original shape...but for some reason, though it returns to round, it does deform in diameter. Eventually it gets to the point where your Climatline hose won't readily hold the mask coupler anymore.
If it hasn't stretched much, a good wash of the inside of the end of the hose can help...it becomes a little more grippy. In my case, that helped some, but every few days the hose would blast off the mask in use. A real PITA.
So, what did I find that works? A small-ish ziptie. Insert the mask coupler into the hose and snug up a ziptie on the outside of the hose. If you do it "just right" you can connect and disconnect the mask coupler at will and it holds it fast when you sleep. If you want (or have a mask that has a detachable elbow like the F10, Quattro Air or FitLife) you can crank it down so it really hold on. After all, zipties are cheap...easy enough to cut the little ones off with a nail clipper.
I do recommend using an Emory board or similar to file down the sharp edges of the ziptie, though....keeps it from snagging the pillow case, scratching you or similar.
Easy solution to save replacing a perfectly good Climateline hose that has stretched.