Diagnosis
This is usually one of two things: weak batteries, or the infrared (IR) receiver sensor on the soundbar is partially blocked or dirty.
How to Fix It
Start with the batteries. Even if they're not dead, they might not be providing enough voltage for a strong signal. Replace them with fresh name-brand batteries. While the battery compartment is open, clean the battery contacts inside with a cotton swab and a tiny bit of rubbing alcohol to remove any corrosion. Also, check the remote's IR LED at the front—make sure it's clean and not obscured by a sticky residue.
If That Didn't Work
Check the soundbar itself. The IR receiver is a small, dark-colored window on the front panel. Make sure nothing—a decorative object, a TV stand lip, even a thick layer of dust—is blocking the path between your remote and that spot. Wipe the receiver window clean with a dry cloth. Also, bright ambient light (especially from some LED or fluorescent bulbs) can flood the IR receiver and drown out the remote's signal. Try dimming the room lights.
Heads Up
These remotes are basic IR remotes, not radio frequency (RF). They require a clear line of sight. If your soundbar is in a cabinet or behind a glass door, you'll have problems. The remote that comes with these is known to be a bit cheap-feeling, but fresh batteries usually solve most issues.