As the previous speakers have already written, it depends on how you transfer the sound to the receiver. HDMI? Toslink? Direct connection BluRay player to receiver or is the TV in between? Are the HDMI cables suitable? Does the signal pass through the TV, is eARC active or were the corresponding HDMI ports on the TV and receiver used? Is the output on the BluRay player set to PCM or bitstream? It must be set to bitstream, otherwise it won't work.

As far as the definition of ATMOS is concerned, I have to disagree with Anonymous. For the benefit of the reader, I will try to explain:
It is not a name or a definition of picture and sound. It is a sound format in itself from Dolby. An additional "track" is added to a Dolby Digital sound track with up to 128 individual sound tracks. This defines where in your room which track should "sound". The receiver then calculates from which speakers, how loud and with what delay the sound must be output so that it sounds there from your seat. It is important to set up the speakers as described in the instructions and to adjust them precisely with the supplied measuring microphone. The more accurate these settings are, the more accurate the result will be in the end.
With a 'normal' 5.1 speaker setup you won't hear much difference. It will only be really good if you either hang 2 or 4 speakers on the ceiling or with Atmos compatible speakers that you can put on the front or surround speakers, which will send the sound diagonally to the ceiling. The sound will then bounce off the ceiling and at the seat it will sound as if the sound is coming from above. However, this is not very accurate and if you have the possibility to hang speakers on the ceiling, prefer this! If that's not possible, I can recommend these: Onkyo SKH-410 (1 pair)
They can also be mounted on the wall and sound really good!

Hope I could help you, otherwise just ask again. Preferably with a description or drawing of exactly what you have wired.

Atmos is a separate soundtrack such as DTS, DD: DTS, DD. The film must contain an Atmos soundtrack. Not every 4K film/broadcast has Atmos. If an Atmos signal arrives at the receiver, Dolby Atmos is also shown in the display. Of course, you also need an appropriate HDMI cable: https://www.digitec.ch/de...

How did you connect the receiver?
From which device is the signal coming?
E.g. directly from the TV to the receiver or from the Blueray player to the TV and then to the receiver?
I had to fiddle around a bit until I figured out how to do it...
With the TV, you can set it so that the sound is not processed but only forwarded. With the receiver, I would try "Direct".
As far as I know, Atmos is only the name for a 4K picture with surround sound. So if you are watching in 4K and have DTS or Dolby Surround, it is actually already Atmos.