Your liquid's probably too thick. I used a lot of Red Oak JC liquids (all VG) and they did this a lot with SmokTech carts.
Take a hammer/nail and make the holes bigger. Don't go in too deep or make them way too wide.
A lot of time VG's so thick it can't pull a steady stream through the hole. It'll pully tiny blobs that get stuck up in there. I have better luck with thinner liquids with those things but still have to punch them.
If you blow in the cart with the tank on, it'll keep draining out until it's almost empty. Best advice it to take the carto out, cover the holes on the side, blow it out, and let it sit for a while. I'll normally just move on to the next cart while I'm waiting.