Outdoor water tap closes with difficulty - see photos

Hello,
we have two identical outdoor water taps (model: Polar II outdoor valve from Schell) at different places on the exterior wall of the house. Over the past 7 months, they have worked wonderfully. Two days ago, I turned on the outdoor water tap and normally watered my lawn with the connected hose. Afterwards, about 10 minutes later, I wanted to turn the outdoor water tap back to the right to close it, but I kept turning and turning, yet the water continued to flow. As a complete layman in craftsmanship, I was quite panicked at first. Then I turned the handle to the right again, this time applying some pressure against the handle (towards the house wall, so to speak). This closed the water tap.

This problem still exists with this one water tap. Therefore, I pulled off the handle on both water taps for comparison and found that on the "defective" water tap a gold-colored part sticks out far (see the picture attached, marked in red). This part is probably called "upper part with spindle." Also, the white plastic part can be turned left and right without resistance, without anything happening. If I press the white plastic part back towards the house wall / towards the water tap with a lot of pressure and turn it, I can turn the water on and off.
For comparison: On the functioning outdoor water tap, the gold-colored part does not stick out and the white plastic part sits very firmly, I can’t turn it just with my hand without the handle.

Now to the question: I have already tried in a layman-like way to "push in" the white plastic part or the gold-colored part with slight pressure. That didn’t work. I also haven’t hammered it in strongly because I don’t know if something might break then..Does anyone have tips on how I can solve the problem?

The white plastic part transmits the rotational force from the silver-colored knob to the shaft. If something slips, it is most likely that the white plastic part is not seated properly. To check this, first pull the plastic part off the shaft (without using force!). Then you can determine whether it is already worn out inside or still has a chance to grip on the shaft. The shaft is not completely round but has at least one flattened spot. Sliding it on should work well by hand; it is a bit tight, the key is that you put it on in a way that it does not get twisted. That’s basically all there is to it. If the plastic part is defective, you can either order a new one or glue it onto the shaft (a worse solution). The shaft should be fine; brass is usually harder than the plastic.

Short addition, I just quickly went home because it’s been bothering me:

I was able to remove the plastic part from the broken faucet. It doesn’t look damaged anywhere. When it’s on, you can move it slightly back and forth, so it has very minimal play in that direction (just like the working faucet – although with that one, the part can’t be turned left and right like with the broken faucet because it fits firmly).

With the working faucet, I can’t remove the plastic with my hands at least, probably because I don’t have the clearance at the back that is present on the broken one to properly grip the plastic part.

When I look into the plastic part from the front on both faucets, it looks to me like they are sitting the same, meaning it doesn’t seem to me that the plastic part on the broken faucet is only halfway on (see comparison photo).
I also can’t push the part further onto the last piece of the upper part/spindle of the broken faucet with normal finger strength. If I push harder, I’m afraid it will break..

Try the defective faucet with the plastic part of the functioning faucet. Then it will be clear whether the problem lies with the plastic part or not.

The effect occurs when the water contains minerals (although it tastes better then). It accumulates inside the faucet and blocks the mechanism. Shut off the main water valve, When the retaining ring (circlip) is removed, the core (movable internal faucet part) can be unscrewed. Clean the parts (if necessary with a copper brush), coat with faucet grease (DIY store) and reassemble. If the handle's inside (toothing) is "zergniesgnadelt": replace it! Done

I am somewhat confused that both plastic parts (which are probably called locking bushings) cover the same amount of the gold-colored part on both outdoor water taps. Sorry for the layman’s terms. So I can also push the presumably defective plastic part completely onto the first water tap, but then it can be turned left and right without any resistance, which is not the case with the other one. However, with the other water tap, the gold-colored part does not seem to stick out as far, which you can see in the first comparison picture, where both plastic parts are pushed on as far/ the same distance as possible. Of course, I don’t know if that is even relevant.

In any case, I earlier removed the rotary handle from the “working water tap” and attached it to the “defective water tap with the possibly defective plastic part.” But the same problem occurs: when turning (whether left or right) nothing happens, there is no resistance felt, and no water flows. The more often I do it currently, the more I feel like I have to press the handle toward the water tap when turning for water to eventually flow.

I couldn’t remove the presumably working plastic part, and also didn’t want to apply full force to it for now.
I will buy new locking bushings tomorrow and will report whether that was the cause.

thanks for the tip. If it wasn’t the plastic part, I will try it that way.