We used to have the same problem with a shower ...
Our shower had twin elements and one had always ceased to function.
When you turn the dial on your shower to increase the heat... are you aware that you are actually decreasing the flow of water across the element ..?
The longer the water takes to passacross the element, the warmer your water will be....But if one element fails your water will be only warm at best... and reduced to a trickle.
...and as Scotman previously mentioned.....When the element that failed was removed it was always ....full of limescale...!