Check the wick in the cart, you may have a wick that is a little too dense, and won't let enough juice flow through to keep the atty happy. This happened to me just yesterday, so I pulled it out and just took a little bit of the "wool" off (make sure to trim away any "fringes") Also make sure to look at the mesh in the atomizer, there may be a strand or two on the wool embedded on that, also. Very carefully with a thin tweezer pull it off if you see some. The cartridge here is key, long as you have heat. Good luck! I keep a little pen-light so can see down in there properly.
Just an added thought, don't "twist" the cart into place, when you do that the bridge in the atty will push the cart filler away from it, and it also can imbed fibers onto the mesh by the twising motion. Straight in, pull straight out.