What brand 12grams are you using? One thing to remember is that unless it's for medical use, the sizing on each one is going to vary a little. Usually if you adjust the marker to work with one from a box, the rest of that box will be pretty similar in size but if you're changing to another box you should check size again first.
Is it leaking from the cupseal area or within the SA-17? Do you have the latest version of the cupseal (Kee/Empire will usually send you the latest version if you ask them).