Several factors can be responsible for the error 'unity hdrp reflection probe not working':
Incorrect Layer Settings: Reflection Probes may not work if the layers they are set to reflect are not correctly assigned or if the objects you expect to be reflected are on different layers.
Probe Resolution and Quality Issues: A low-resolution setting for the probe can result in poor or non-noticeable reflections.
Volume Settings Misconfiguration: In HDRP, reflection probes work within certain Volume settings; misconfigured settings can lead to the probes not functioning correctly.
Static/Dynamic Mismatch: If the scene objects are dynamic but the Reflection Probe is set to Static, it won't capture real-time changes in the environment.
Shader/Rendering Settings: Certain shaders or rendering settings may not fully support reflections or require additional setup for reflections to appear.
Culling Mask Configuration: The Reflection Probe's Culling Mask may exclude certain objects from being reflected due to misconfigurations.
Solution - Here's How To Resolve It
Adjust Layer Settings: Ensure that all intended objects to reflect are in the correct layers as specified in the Reflection Probe's settings.
Enhance Probe Quality: Increase the resolution of the Reflection Probe under its quality settings to get better reflections.
Configure Volume Correctly: Adjust the related Volume settings to make sure they are not hindering the function of the Reflection Probes.
Match Reflective Object Settings: For dynamic objects, use a Realtime Reflection Probe, or for stationary objects, bake the reflections with Static Reflection Probes.
Update Shaders/Rendering Settings: Check if the materials/shaders of the objects that need to reflect light are compatible with reflections and HDRP settings, making adjustments as needed.
Fix Culling Mask: Configure the Reflection Probe’s Culling Mask to include all the objects you want to have reflected.
For more specific issues or complicated scenes, consulting Unity's official documentation on HDRP and Reflection Probes, or reaching out to the Unity community forums for additional help can also be beneficial.