My heat pump works fine in the air conditioning mode. When in the heat mode it works fine until it goes into defrost mode. It works fine in the defrost mode but occasionally does not want to come out of the defrost mode. You can tell it is in the defrost mode because of the buzzing sound and the external fan is not working. I know that refrgerant is critical but is there anything else that could cause the problem

Depending upon the defrost control style it could be the sensor that tells the defrost board what temp the coil is at or the entire board.