Immersion water heater not working?

We live in a rented flat with electric heating. In February the immersion heater on our boiler packed up and our landlord had it replaced. However, just 2 months on it seems to have given up again. The dedicated power supply still has its light on but water from the taps is luke warm at best and the boiler itself its cold.

Luckily the shower and the washing machine heat themselves but we're concerned that the immersion heater has given out again so quickly. Why might this be happening?