I bought one of these 6 months ago. I rarely switch to shower mode. Several times the part was quite difficult to operate, but could be made operable again by moving it.
Then last week it stopped working :-(
Well, we have quite hard water here in Frenkendorf, around 30° fH!
So I dismantled the thing and soaked it in descaler.
Even after two days in the descaler, it still wouldn't move :-(

So I decided to get to the bottom of the cause, after all I am curious.

So I dismantled everything as far as I could. There was no more limescale to be found, so the descaler had done a good job. But even with force I couldn't move the part. Even WD40 and leaving it to work for a day didn't help.
I then decided to saw open the shovel ring to get to the inside.

Well, and then it was clear!

The O-ring had seized up with the plastic.

The problem must be in the plasticizer area.
This is actually a known problem in engineering, but can be solved with the right choice of material!

Yesterday I bought a new part.
After unpacking it, I noticed that the first time I moved it, the O-ring was a little tight, then it was easy to move. But after just a few hours, I had to use a little more force to move it for the first time.

I have now greased the critical area (cock grease, because it comes into contact with drinking water!).
Let's see how it is in a week's time if I don't move it until then.

BTW: I previously had a similar part from ALDI with the same problem. This also stopped moving after a while, but I didn't have time to dismantle it and get to the bottom of the cause.

Conclusion: It is a design or manufacturing fault!
If the material pairing was already chosen in this way during construction, it is a design fault.
However, it is also possible that the pairing only occurred during purchasing because material specifications (plasticizer) for the O-ring and the plastic were not observed.
It is also possible that greasing avoids the problem, in which case the fault would be found during assembly.

MfG Peter(TOO)