I've never been able to figure out the trick of drying off the carboy rubber stopper after soaking it in starsan - it always seems to work it way out of the glass neck, so lately I've just kept a 1 1/2" blow off tube for the entire 7-10 days before moving things over to a corney keg. Is there a snap clip that would help the rubber plug stay in place?
Sometimes I can persuade stubborn rubber stoppers to seal by wiping the contacting surface with a paper towel soaked in rubbing alcohol. Let it air dry for a few minutes and twist it into a sanitized, but dry, carboy opening. My guess is that the alcohol displaces the water and dries the rubber surface, leaving it slightly tacky and ready to grab the glass better.