A difficult thing for me was getting the clay off the cork (yes even with newspaper around it). Although another hard aspect was glazing the inside, the most challenging thing was getting the lids to fit right. As you can see below, the green vessel's lid is too big and does not fit correctly.
I think the most successful of my jars was the red, middle sized one. The type of lid for this one was flat with the locking aspect on the inside around the interior border. This is most successful because the lid actually fits and the glaze is equally covering the jar. The reasons my other ones were less successful was that the green jars lid turned out to big and not symmetrical and the purple jars lid became too small and not a perfect circle ( so its tricky to get the lid on right).