Why my heat pump only heat in emergency heat only,the condens is cutting off and on

It sounds like a maintenance issue with your heat pump, its clear that the unit is trying to run but one of the safeties is shutting it down trying to protect itself. most heat pumps are equipt with a High press lockout usually a manual reset ,a low press, cut out switch resets on rise of press automatically. Causes of LPCO Extremely dirty condenser coil or evaporator coil, Loss of fan on condenser, Low refrigerant charge. A competent Service man should be able to locate and correct this problem. A seasonal check up on the unit avoids these problems.