wont heat after defrost

by craig
(danville, pa)

I have a Nordyne heat pump model jt5bd-030k 2.5 ton and there is no heat after defrost sometimes it don't happen all the time . I found that if I turn the thermostat to cooling for about 30 seconds then back to heating it will work fine for a couple days and the temperature has been low of 20 f night to a high of 34 f during the day