I use them all the time and love them dearly. A few things to watch for.
First, don't fill the tank all the way to the top. Leave about a 1/16th or even 1/8th inch airspace at the top of the tank. I've had the flow lock up when I've completely filled the tank.
Second, no dark juices, if I can't see through the bottle, I use different style of atomizer.
Third, with a new carto, I put it in the tank, fill it, cup the end of the carto with my palm to prevent air getting in and suck on the other end until there is a drop of juice in my hand. You should see the juice level go worn in the tank. New carto? About 1/4 inch drop in fluid level. Set the tank on its' side for about a minute or two. Look in the drip tip end. Do you see the filler looking wet? No? Repeat the process. Usually takes me two cycles to get it wet looking. When it looks wet, let it sit for a minute or two. Stand it up on its' 510 connector (I use pill bottles to hold my cartotanks) for 5 minutes or so. Top it off, leaving an airspace.
Vape at 8 watts. Don't fire the carto longer than 8 seconds and draw at about the same rate that you would drink through a straw. Enjoy the lovely flavor and vapor. Repeat as necessary until you run out of juice.
You may or may not hear the carto crackle. Seems to depend on the juice and the age of the carto so I don't use it as an indicator.
Chain vaping? Let it sit upright for a few seconds at least between draws.
While you are vaping, you should see bubbles coming out of the holes in the carto between draws. Varies, a small bubble or two every now and again. Do not let the fluid wicking holes at the bottom of the carto be dry. Make sure that you don't tip it too far up in the air. If the holes get dry, the carto will burn.
If you get a burned taste, there is little chance it will ever improve. I throw mine out, they are about a buck apiece from here: vapelite.info I buy the F16 version, flanged and pre-punched with two holes.
Hope this helps. My cartos last about 30-40ml of juice typically. Good luck and have fun!