There are a number of posts calling on Roku to fix the lack of Atmos on Netflix and Amazon Prime. I don't get Atmos from either service but I do get it from Disney+
I spent about an hour on a support call to Netflix a couple of days ago and the concluded my setup is not compatible - they had no response to my, "but it works seamlessly with Disney+, I don't need a higher subscription for extra money (Netflix), I don't need to search for multiple versions of the show (Amazon) - it just works. Available shows display the Atmos flag, and the Amplifier displays, "Dolby Atmos" while playing"
I have a feeling that they are tying UHD video to UHD audio if you don't have 4K video you won't get Atmos.
Roku Ultra Integra Amplifier Sony Projector All device connections are HDMI Internet is Ethernet with CenturyLink 1GB service
Seems to me that Netflix & Amazon need to upgrade their services to support capable configurations. I intend to post this comment as a support ticket on both platforms.
It is Netflix due to how they transmit Dolby Atmos audio. Rather than send it as metadata embedded within a Dolby Digital + audio stream (which is how Prime Video, Vudu, Movies Anywhere, and Apple TV transmit Dolby Atmos) as pass-thorugh audio to your receiver, Netflix wants the streaming device to decode the audio and send as PCM + Atoms to your receiver.