Diagnosis
This is usually one of two things: weak/bad batteries, or interference from other devices (especially LED lights or soundbars) flooding the room with infrared noise.
How to Fix It
Start with the batteries. 1) Take them out and lightly clean the battery contacts in the remote with a dry cotton swab. 2) Put in brand new, name-brand alkaline batteries. The cheap ones that often come with electronics don't last. 3) Re-pair the remote. Go to Settings > Remotes & Accessories > Set up remote buttons. Follow the on-screen steps—this ensures the Bluetooth connection (used for some functions) is solid.
If That Didn't Work
Check for interference. Turn off any nearby LED light strips, lamps, or soundbar indicator lights. Also, make sure there's nothing physically blocking the TV's IR receiver (a small dark window on the bottom bezel). Try using the remote from an angle, not just straight on.
Heads Up
The remote uses both IR (for power/volume) and Bluetooth (for navigation/voice). If voice search works fine but volume doesn't, it's an IR-specific problem. If nothing works, the remote itself might be faulty.