Solar water heating.

It isn't clear to me from your post what kind of solar water heater you have. We have had a solar water heater installed for 2 years now. Ours are the model that has what looks like standard water heater turned on its side with glass tubes coming out of side. It has provided our guests hot water(we rarely if ever use hot water ourselves) in rain or shine. In cloudier weather it does take longer to recharge but we have yet to run out of hot water. One thing that we have done is once a year have someone come out and flush the system because of the extremely hard water in the area causes significant mineral build up if it isn't maintained. Perhaps that is the problem with the unit where you are?