Certainly refractometers are easiest and fastest but a clean, calibrated and appropriate hydrometer is more accruate. To be clear I mean a long glass cylinder hydrometer, not the plastic swing arm type found in Capt. Crunch cereal boxes.
Are you reading the glass hydrometer correctly? Make sure you are reading at the lowest level of the Meniscus.
http://en.wikipedia.org/wiki/Meniscus
What model of hydromter are you using? It will be somewhere on the paper within.