Pressing the power button briefly wasn't causing the tablet to go to sleep. I did a factory reset and immediately afterwards both pressing the power button and the timeout setting caused the tablet to go to sleep. After a short while both methods stopped working again. Shutting the tablet down overnight brought the sleep function back for a short time, but then it stopped functioning again. Did another factory reset, but that failed to restore the sleep function the second time. Someone suggested in another forum for a different android device with the same problem that it might be a defect in the power switch. Anyone have any other ideas? Or any advice about the possibility of replacing the switch? I did have Mantano Reader configured to never sleep, but I think the factory reset should have eleminated any problem caused by that. Wouldn't it?