It may take 1 more battery inside, Usually 2 - # 377, AG4, # 377A or SR626sw ( all the same ) Also you may have to move the 1 battery around even though it is a tight fit and put a little piece of tape on it when it works and put a little extra folded to cause enough pressure on the battery. Is the back round and flat or does it have 6 little notches around the edge of the back watch cover ?