I have the same problem. The remote works only when connected to a USB charger. The screen shows that the remote is charging. However when the charging cable is removed, (even after letting it charge for a whole day) the remote shows almost no battery life. Sometimes, it shows a tiny bit of charge. But it only lasts for a couple of hours.
If you're having issues with your Roku Voice Remote Pro, we would recommend starting with the troubleshooting steps available in this Support link: How to resolve issues with your Roku® simple remote or Roku voice remote. If none of those does the trick, please reply here with the serial number of the Roku device associated with the rechargeable remote. It can be viewed in Settings > System > About.