If you have the original converter, use the converter and see if problem persists.
Waterman had two kind of cartridges, long and short CF, which fit perfectly well on all older pens, and short standard international, which do not provid a tight fit. It may be the case that your pen has a CF format nipple and in that case, the international standard is not adequated. Currently, per waterman site, all waterman carts fit on watweman pens. Whereas they are proprietary or international, not a word from the brand. AFAIK, they no longer make CF cartridges.
If problem persist with original french converter, then your pen’sproblem is not related to carts .
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