Ah, yes, Watson, with the scantiest of clues, the great sleuth will find out all that could possibly be the cause of this problem.
The facts given: Some rooms on the first floor are cold, while others are hot. We know that the heating elements should have hot water in them to heat the rooms. If the other two rooms on the same floor have water in them, then there is probably water in the other two rooms.
Is it possible that a zone valve has failed that would let water pass to the other two rooms? Probably not, it sounds like a cheapened system. Is it possible that the system was installed incorrectly with a loop that is too long? Possibly. Is it also possible that the system is a one-pipe system with the other baseboard loops full of air? Probably. Could we go on with no other information? NOT! How many angels can dance on the head of a pin is the region of other philosophers.
The next chapter brings a prolonged monologue of expertise in hot water heating developed over a lifetime. Keep tuned.