Lately my furnace has not been cutting in when it is supposed to. It used to be alright all day long but during the night when it dropped back it would not restart in the morning.To get it restarted-I discovered this by accident all you have to do is lightly tap the burner itself and it would be o-kay. Now it is happening almost all the time.It is fine when it gets going but when it cuts out on its thermostat setting it won't restart until I give it another light tap. I checked all wiring nothing loose. Any ideas?