Hydrometers can really be a piece of crap, they are hit and miss and then depend on being clean and free of salt buildup. Also any bubble under the swing arm with throw off the reading.
I would highly suggest a refrectometer, not only are they very accurate but incredible easy to use and maintain. Way worth it.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.