Like anything on a boat, these fittings need regular maintenance and attention.
With the wet weather in the Bay Area, we advise clients to use lanacote (avail. at west marine) on the fitting threads to prevent seizing every 3-4 months. Also, please note that aluminum deck fittings are problematic and tend to seize much more often than other materials.
If a client has a seized fitting usually we advise them to use a penetrating oil and lightly tap the fitting with a hammer, then use a little more penetrating oil and let it sit for a day or so. repeat the process until the fitting opens. This can take as little an hour, or as much as a week.
Our techs are instructed to only tighten these fittings until snug - i.e. "finger tight". This is to ensure they can open the fitting on the next visit. The techs are also instructed to never lubricate a fitting for any reason to make sure that deck-fitting areas are never left potentially hazardous for our clients.