Smells in the bathroom - possibly sewage pipes not laid correctly?

Hello experts,

we moved into our newly built house about 2 years ago - and more and more unpleasant odors are developing in our bathroom...
The question is whether the wastewater pipes might not be installed correctly, perhaps with too little slope?

I have attached a file with this name: BILD - BadWCKasten + Text Dec12.jpg, showing all the pipes and labeled accordingly.
Can someone tell us something about this?

Thanks!

I don't want to be indiscreet, but how often is the shower used?
Which drainage element was installed?

...the shower is used about every three days (we still have a second shower on the ground floor), I can’t say anything about the drain element... apart from the fact that we have a channel with the drain / siphon etc. in the middle. What should I look at to determine if the shower drain could possibly be a problem?

I would have spontaneously guessed a dry siphon in the shower, but if there really is water flow there every three days, I can hardly imagine that. How is the bathtub drained?

hm - found a picture...you can see that under the tub, before it was installed, there is a large connection in the floor, presumably for the wastewater, when the water is drained from the tub. I can't see 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, for example, leave a still quite wet dishcloth spread out in the kitchen overnight, it is dry the next day, meaning: in our house, wet things dry quickly, perhaps the water also evaporates quickly from the traps, and maybe three days are really sufficient?

Oh dear, I know that problem! Actually, it should be fixed when new pipes are laid.