I think a very slow and gradual loss of suction is actually natural and normal, unless you physically cut off the supply of air to the tubing. Like if you bend the tube shut or use an airlock.
If you're losing suction quickly, it could be your position or the size of your chest. Heck, it can even be the lubrication you use! I used to use olive oil as a sealing lubricant, but ugh, the side of my right breast would always have a small fold when noogling and air would happily slip right in! Oil was a good sealant for me.. but not for folds!
BUT I decided to use a water-based lotion and that did the trick! Although your skin will absorb the lotion, it does plump up your skin with moisture temporarily and that fixed the fold issue! The fold is still there, but my skin's cells are plumped up enough to not provide an airway for suction to go bye-bye. ;D
You can also hold down the cups towards you if you want to be a bit more secure. And always put the cups as close together as you can! Tying them together with a cloth, bra, band, etc. really helps!
