Quit reusing cartridges so long. They are getting overly saturated. Take tweezers or a tooth pick, pull the foam out and wring it out in a paper towel. Than, reinstall and refill. It seems that if you keep "topping off" cartridges, they get to a point they cant draw properly. I have a post somewhere here looking for bulk 510 empty cartridges with the filling. They shouldn't be more than a nickel each, but everyone wants high dollar for them..
Also, as someone earlier suggested, blow your atomizers out occasionally. You cant suck them dry. I originally thought that if I kept puffing, they would eventually clear out. Not so...