I live in a 2 family flat (upstairs) with radiator heat, Question, why do some of the radiators work sometimes?

and then sometimes the other radiators will work? There does not seem to be any pattern in this. Also the boiler (over 20 yrs old) loses half of its water a day and I can not find ANY area where there is escaping water or steam in the part of the system that is not in the walls.HELP!!