A lot will depend on how long you are stopping in JFK. If it's less than 24 hours, then it could be married segment inventory issues (and is likely trying to book segments on single fare component). Anything longer than 24 hours will break fare (you'll end up with separate fares for LHR-JFK and JFK-LAX segments) and there is no married segment inventory applicable. You might run into fare combination restrictions when booking fares on single ticket and there is a fare break.