DIAGNOSIS The audio output setting is incorrect, or an intermediate device does not support Atmos passthrough. Atmos requires specific HDMI and settings chain.
FIX Configure the audio settings in sequence. Step 1: On the Fire Stick, go to Settings > Display & Sounds > Audio. Step 2: Select 'Audio Output' and set it to 'Best Available'. Step 3: Select 'Dolby Digital Plus' and ensure it is set to 'On'. Step 4: Most critically, go back one menu and select 'Audio Video Sync'. If this is set to anything other to 'Off (Recommended)', change it to 'Off'. This setting disables audio processing needed for Atmos.
IF THAT DIDN'T WORK The connection chain is broken. The stick must be plugged directly into the soundbar's HDMI IN port, and the soundbar's HDMI OUT (ARC/eARC) must go to the TV. If plugged into the TV first, your TV must support eARC and pass through Atmos, which many do not. Check your soundbar's input mode is set to the correct HDMI port.
HEADS UP Only specific apps deliver Atmos. Test with known Atmos content: the 'Atmos Demos' section in the 'Dolby Access' app from the Amazon Appstore, or certain titles in Prime Video and Disney+. Netflix requires the Premium plan. Hulu does not support Atmos on Fire TV.