What unit do you have, that would be also helpful...
even with a full cartridge, sometimes it's not making good contact with the 'bridge' in your atomizer. Make sure you're not twisting the cart in and out, push in straight and pull out straight, also. Start by filling the cart till just before you get any 'puddling'. Then check the position of the bridge in the atomizer, when it's fully engaged with the battery. Insert the cart straight in, in line with the bridge, so you have the widest plane of the cart lined up with the direction of the bridge. This sounds silly, but with an 801, for example, there are two different style bridges, one that is realatively flat, the other the one that looks like an inverted "V". The LATTER one is the most important to make good contact, I hate them personally. They seem to push the cart filler away from themselves. And the 'carts' have a definite wide side and a narrower side. You want the wick and bridge to line up, you don't want the plastic part of the cart to touch, just the wick!!
Ok, so all juiced up, insert, and see if it helps at all. If not, pull the cart back out and pull the wicking just a bit out of the holder, and reinsert. Straight on, straight off. May be you're not puffing long enough. You need the atomizer at top heat and length of time for good vapor.
If you have good vapor when you drip onto the atty, I would say it's a good chance your cart wick is not making good enough contact. And wait a while to take your next puff, sometimes the wick will get too dry at the tip if you over-do puffs per minute. Give it a chance to re-generate more liquid.
I use an 801 atty and cart, I always use a needle bottle to re-fill. It fills from the bottom up, rather that dripping
juice in from the top down. Give it plenty of time to soak up as much juice as possible without getting puddled up (too full). Too full can cause flooding of the atomizer. If it floods, take if off, blow it out from the thread end, hard, and start over.