I had the same issue with my laptop some days ago. At the moment it seems to be working most times though, no idea why. I only made some seemingly unrelated changes in BIOS settings (fan speed, battery charging). The screen went blank, came back a second (as it usually does when about to hibernate) and went blank again. After that the laptop would still not turn off until the battery was fully drained. If I turned it off manually after the screen went blank the second time, however, it would later still resume all right. I guess it's something about triggering the correct acpi-event for poweroff or something.