Possible reasons for the fact that the flux of the solar water heater in the outlet is too small:

The solar water heater is not assembled at a high position. The vertical gap between the heater and nozzle is not large enough (especially on the top ground). The way to solve this problem is to increase the height of the heater or enlarge the diameter of the pipes. 

Why dosen't the solar water heater heat the water efficiently?

Local air quality: Dust covers the tubes, which reduces the absorbing rate of the evacuated tubes.
Local water quality: Furring of the tubes, which reduces the efficiency of heat transferring.