ᐅ Odors in the bathroom – possibly due to improperly installed wastewater pipes?

Hello experts,

We moved into our newly built house about 2 years ago, and we are increasingly experiencing unpleasant odors in our bathroom.
The question is whether the wastewater pipes might have been installed incorrectly, possibly with insufficient slope?

I have attached a file named: BILD - BadWCKasten + Text Dec12.jpg, which shows all the pipes clearly labeled.
Can anyone provide some insight on this?

Thank you!

I don’t mean to be intrusive, but how often is the shower used?
Which drain fitting was installed?

...the shower is used every three days (we have a second shower on the ground floor), I can’t say much about the drain element...apart from the fact that we have a channel with the drain / trap etc. in the middle. What should I check to determine if the shower drain might be a potential problem?

I would have initially guessed a dry trap in the shower, but if there is water flow there every three days, I find that hard to believe. How is the bathtub drained?

hm – I found a picture... it shows that underneath the bathtub, before it was installed, there is a large connection in the floor, presumably for wastewater drainage when the water is emptied from the tub. I can't see much more than that...

Assuming the traps dry out, are there 'deeper' traps that hold more water and therefore don’t dry out as quickly? I have noticed that if I leave, for example, a fairly wet kitchen cloth spread out overnight, it is dry the next day; meaning in our house, wet items dry quickly, maybe the water in the traps evaporates quickly as well, and perhaps three days is really enough?

Oh dear, I know that problem! It should actually be resolved when new pipes are installed.