Maybe you should check the hoses that go through the bulkhead to see if they get hot - the matrix is fed all the time, a flap inside the heater directs the air through it if heat is needed so if either or both are cold then there's either a blockage or an air lock. If they're both hot hen you need to check this flap, they're run by a similar motor to the recirculation flap but don't seem to fail anything like as often.