Reason: There might be some obstacle or obstruction that does not allow sunrays to fall on the vacuum tube of your system.
Solution: remove the obstacle and make sure that sunrays do fall on the system through the day.

Reason: 1. There Silicon rings(Washer) beaked or inserted incorrectly.
              2. Crack in water inner tank nipple.

Solution: 1. Change the silicon ring or install it again to let the tank and solar tube properly.
              2. Change the damaged nipple.

Reason: 1. Mixer tap problem in the bathroom.
              2. Locking of Float ball is not done properly.

Solution: The mixer tap should not be kept in the centre position, because it will create reverse flow of cold water into hot water pipe which will affect the overflow of water from air vent.