Make sure you have a premium Netflix subscription, and log in to your Netflix account on a web browser and set the Playback Settings under your Profile to HIGH. Also be sure you are playing the original language track. But is it possible that Dolby Atmos isn’t supported on Netflix on your Fire TV Stick model.
Prime Video has VERY limited Atmos content. Another film on Prime Video that has Atmos audio is The Tomorrow War.
There is a good chance Dolby Atmos isn’t supported on Disney+ on your Fire TV Stick model. If you want Atmos support on all the major streaming apps, switch to an Apple TV 4K, Nvidia Shield Pro, or the latest model Roku Ultra.