One problem I have found with covers, is that the "hinge" part stretches and the front doesn't moves and the magnet doesn't line up properly. The cheap cover I had on my Aura H2O was particularly bad. Unless I carefully lined it up, the device didn't go to sleep.
My suggestions would be:
- Reboot the device every few days. It could be a rogue background process.
- Take it out of the cover for a while and rely on the auto sleep.
- Manually reinstall the current firmware. There have been some issues over the years that this seems to solve. It shouldn't help, but it seems to.
- Do a factory reset.
Other than that, it is talk to Kobo and get it replaced.