Diagnosis
The IR receiver sensor on the projector is likely obstructed or has a very narrow reception angle. It's located right next to the front lens.
How to Fix It
Remove any clear protective film you might have left on the projector's front face, especially around the lens. Ensure no physical objects, like a soundbar or a cable, are blocking the immediate area around the lens.
If That Didn't Work
The remote's IR emitter might be weak. Try using your smartphone camera to check the remote—point the remote at the phone camera and press a button; you should see a faint purple light through the phone screen. If not, the remote is faulty.
Heads Up
The IR sensor on this model is notoriously finicky. For reliable control, consider using the BenQ Control app on your smartphone over WiFi, or get a universal remote that supports BenQ codes.