Try a different USB port, wall adapter, or location. If it charges from one source but not another, it's a voltage/power-source issue, not the watch.
Avoid high-wattage fast-charging bricks (20W+ USB-C/PD wall chargers) — these can trigger a safety shutdown on the watch's charging circuit. Use a low-power source instead: a standard computer USB port, an older 5V/1A wall adapter, or a low-wattage power bank.
Use the original CARBINOX magnetic charging base and a 5V/500mA-or-higher certified power adapter. Off-spec third-party chargers are a common cause of failed or inconsistent charging.