It could be a problem with the heating, it could be the switch (try flipping it faster or slower), but it could also be that the water pressure is too low.
In the latter case, you will have to top up the level.
Check the documentation for your heater. Mine is the Heatrae Sadia Electromax and its pressure needs to be above 1 bar.
See the black “hand” indicator in the image below? That is the pressure. Other heaters can have two zones marked red and one marked green between which your pressure needs to be.
Some heaters will stop working and activate an alarm light if the pressure gets too low. If it happens in the winter, you may wake up in a cold house with no hot water for your shower.
Here below you see the filling loop that allows you to top up the water level and the two black valves – indicated with a blue arrow – that you will need to flip for this to happen, for this particular heater. Other heaters will likely have something similar.
In this case, the loop is behind the lower panel, which you will need to take off. With other heaters, the loop may be located higher on the heater.
I’ve encountered the low-pressure problem before in England and letting agents and landlords usually have no idea what to do about it. Calling the helpline for the heater does not necessarily get you anywhere either and the manual may actually have been written for maintenance and repair technicians rather than for consumers.
When you flip both switches, you will hear the water run and you will see the pressure go up, slowly.
In this case, it needs to be between about 1 and 1.5, generally speaking. (If you run the central heating, the pressure may go up higher.) Leave any other valves alone!