3 electric showers defective in the same fashion. What gives?

[silverblack]

Check the bore size of the hose from shower to the head,bigger the better for max flow.

 

[Lopéz]

The only thing that wasn't really changed was the shower head. I was using an adjustable one with different spray patterns so I bought a slightly smaller adjustable shower head. Is it possible for adjustable shower heads to cause issues like this ?

Yes - I had a Triton power shower and a sparky broke it when removing the cover to do an electrical inspection.

He replaced it with a newer model but left the old head and hose as the new one didn't match the rest of the bathroom fittings. It did not run well at all, and after changing to the correct head and hose it worked perfectly.

So, not out of the question that it's the head.

 

[Mysterae_]

Is it possible for adjustable shower heads to cause issues like this ?

Definitely can. As part of a their energy saving initiative Home Energy Scotland gave a few freebies and one was a water saving shower head. It was crap! It certainly saved water but turned a race horse's lash in to a OAP's urinary sample flow. It wasn't fitted for long!

 

[kindai]

The fact they seem to work a few weeks and then the flow drops suggests to me the inline filter is blocking up.

The other thing to test is to make sure the temperature wheel and the mechanical bit inside are correctly aligned, eg make sure both are all the way cold before installing it.