Two things to consider...
Is the leak coming from around the outside base, or through the hole in the bottom of the Carto?
If it's coming through the base...attach the carto to a battery so you can work with the carto...gently rock and pull the outer sleeve off and wrap the base a couple times with teflon plumber's tape...or mix up a bit of food safe glue using flour or corn starch and smear a bit around the base. Put it back together...problem solved.
If it's comming through the hole in the bottom of the carto...
First, make sure you aren't accidentally dripping in the center tube when you fill. This is very easy to do.
Next, if you happen to have something like an old KR808D-1 carto cap with the little plug built in...put that on the battery end of the carto before you fill it. If you don't have such a carto cap...use something like a match stick or tooth pick to plug the bottom hole while filling. What is happening...is when the top cap is pushed on...it shoves enough air into the tube to force a few drops of juice through the wicks and out the bottom hole. Capping the hole before filling creates a vacume and helps prevent that. Once you have the top plug back on, the vacume is somewhat stablized, it'll be hard to get a free flowing leak started again unless you have super thin liquid.
Before putting it on your battery...give the battery end a good forceful blow...with the mouthpiece end held to tissue or something. That helps make sure the atty isn't flooded, and insures no gurgling if a little bit of juice did force through when putting the cap on and removing what ever you plugged the battery end of the Carto with while filling.
If it still leaks through the middle...carefully remove the top cap, and push on the inner tube with something like a pencil or pen. Sometimes the tip cap fits tightly enough around the inner tube that it pulls the center tube up a bit leaving too much room around the wicks. Pushing that back down closes that gap some.
If it still leaks after all this...the wicks are not aligned properly. You can pull the carto apart, take out the filler paper and throw it away, take off the center tube, fluff the wicks gently and reseat it all back together. You'll see how it works...two little wick areas...thicker juices need more room...thinner juices you want the center tube pushed closer to them.
Good luck and let us know how it goes