I can think of only 2 reasons a Superfit would not stay closed.
Adjustment is first. By properly adjusting the closing tension by
reshaping the bottom section, it should stay closed. If it is too
loose, or otherwise misaligned, it will open. The other problem is
if the top section is too light to hold the tension. This would
result from letting the original ring to be the entire structural
element on the top, which in some cases is incorrect. IMHO, these
are one of the best of the arthritic shanks, at least the one I like
to recommend most often. I haven't had any trouble with one yet,
though some have required creative installation to maintain
structural integrity and therefore proper fit and operation. If all
else fails, contact the company and see if they have any ideas (I'm
assuming you don't have the installation instructions in front of
Jim in NC mountains, where the ski season is done and spring is trying
really hard to arrive.