It is going to be difficult to tell where the problem is. First you need to figure out if there is a problem.
The heat to the tube will depend on room temperature to water temperature. Is your room close to 25C? If so the tube may or may not heat.
Rain out is caused by warm moist air going through a colder tube, this is why they heat the tube sometimes. If the room, so the tube, is around the same temperature as the water from the humidifier, then depending on very small changes in room temperature the tube may or may not be heated.
Try turning up the tube temperature when you are not sleeping and see if the tube gets warm and stays warm for around 30 minutes. If it does than your system is probably working fine.
Now if you do need to "fix" something, changing the tube will cost much less than changing the CPAP, invest in a new tube and see if it starts working. If not c=have the CPAP checked.
WELCOME! to the forum! and much success to you with your CPAP therapy.