Diagnosis
Expected behavior. The JBL Bar 300 is a 5.0-channel soundbar. It has no physical rear speakers. It uses side-firing drivers and psychoacoustics to create a surround sound effect.
How to Fix It
This is not a fix, it's a specification. For true discrete rear channels, you need the JBL Bar 500 or 700 with detachable battery-powered surround speakers.
If That Didn't Work
Ensure you are actually receiving a Dolby Atmos signal. Check your TV's audio output settings. Set it to 'Bitstream' or 'Passthrough', not PCM. The soundbar display should show 'DOLBY ATMOS'.
Heads Up
The virtual surround effect is highly dependent on your room layout. The soundbar needs clear space on both sides and should be centered for best effect.