This one is pretty common with many robot vacuums including manny models.
The problem is often caused by dirty charging contacts.
Because the contacts are located under the robot vacuum, they easily get covered by dust, lint and other bits of debris.
Cleaning the contacts with a dry cloth should solve the problem. If it doesn’t work try using an abrasive pad or steel wool.
If that still doesn’t work you might have a stuck charging contact or the battery may be faulty. Contact the company’s customer support for advice.