I think part of the problem may be that you're not topping the carto off. If you let the prefilled carto "run out" or vape until it's nearly dry, you're bound to scorch your carto.
Try topping off the carto with a few drops (start with 2-3) after the equivalent of smoking an analog Keep the filler wet enough that it looks "slushy", as SubSplat said above - but not overly filled with free liquid on the surface as you don't want to flood it. Each time you top off, try doing a "thermometer shake" to really get the juice down in there; it aids in the wicking process.
Another thing: although it may seem like you have been very careful in not getting any liquid into the center hole, somehow it just happens! I like to use dental picks made by GUM in a multi-pack that are cheap and have a little snap-shut case. They have white handles and the top part is a green flexible point with little nubs on it. You'd be surprised when you push it back and forth in the hole the dark gunk it pulls out--- and then the carto tastes better and vapes much more freely.