DIAGNOSIS The remote uses RF (Bluetooth) for most functions, not just IR. Pairing can get glitchy, or there's interference.
FIX Re-pair the remote. Remove the batteries. Unplug the TV from power for 60 seconds. Plug the TV back in. While it's booting, reinsert the remote batteries, then press and hold the Return and Play/Pause buttons together for 5 seconds, pointing at the TV. You should see a pairing prompt.
IF THAT DIDN'T WORK The remote's battery contacts might be weak. Ensure you're using fresh, name-brand batteries. Clean the contact points in the remote with a cotton swab and a tiny bit of rubbing alcohol.
HEADS UP This remote also has an IR blaster for power/volume. Ensure nothing is blocking the IR sensor on the TV (a lower front-center white dot). Strong sunlight or other IR sources can interfere.