I have MI TV 75", which has the capability to output atmos. However I didnt get atmos in the first try, and had to learn the tips the hard way. Here are few pointers, which maybe helpful
Ensure you have enabled atmos option in your receiver
You should have atmos speakers connected

The cable from your source to receiver should not be too long, try to limit it within 8 feet
Have a good broadband connection, if possible with atleast 50 MBPS speed
Enable options on your source (Nothing to be done in fireTV though)
Ensure your receiver is turned on, before your source. By that way, sync will be more effective
Not all content in Netflix and Prime will have atmos. Only selected ones has, so ensure you are playing something with atmos content
Start playing, only after the sync between source and receiver is complete
And finally, if atmos is not getting detected. Please shut down all equipments and retry