only 30 sec hot water and goes very cold please help@_@

I just rent this flat but when I taking shower, it seems no hot water at all actully, but the store is hot that I can feel, and the top tank is half full. So I tried to switch the water heater off and then on, and I get hot water. But I dont think this is a good idea for long-term renting.
P.S my green lights outside shows my system is fine. But I can feel the plumber which should offering hot water very cold.

Can anyone help. I cant take any shower now.
thank you.....

Not sure if you've sorted this out by now, but this sounds like a heat sensor problem.

Turning the appliance off then back on often re-sets a failing sensor and it works again briefly. New sensor(s) needed.

Cheers, Mike

The sensor is most likely causing this problem. But wait!, is it only the one shower that is causing the problem? If you seem to have hot water everywhere else except the shower, it may be the shower cartridge that is faulty.