On occasion, the roof top does not fire up - on wet days, sometimes cold days - we have changed so many parts that I am thinking it is electircal - one part we have not changed in 12 years is the wire to the thremostat - thermstat is new as well - I use to be able to turn it off then on a few times and it would fire up - not today and its freezing - 3 different tech have 3 different solutions but none of them fixed the problem - they seem to get it going, leave, and a few days later it starts acting up again.

Its hard to say not knowing the model of the system. It may just need a good cleaning.