Sounds like "White Rust".. In other words, your parts are "oxidizing" shortly after being plated. Maybe not plating long enough.. low amps, etc.
That said, try a mild acid "Bright Dip" (3% Muratic or Battery acid.. 0.5% Nitric if ya got it). 30-40 seconds in the dip, max... Do a "Double Rinse" after the acid dip (two separate containers of distilled water) Upon removal from the acid dip, hold over the acid dip bucket and spray the parts with distilled water from a spray bottle, then drip then douche parts in the first rinse, drain then douche in the second rinse.. "Force Dry" if you can. I use a food dehydrator set at 145 degrees (F) and "cook" my parts for 20 minutes.
If you aren't already doing so (and it sounds like you aren't), ya might want to Clear Chromate the fresh plated zinc parts.. The Chromate protects the zinc from oxidizing.