Default Auto-unlock works but auto-lock doesnít? bad sensor?

Iím using an origami case with the tablet. The flap unlocks the screen just like it should when I flip it open. When I close it though, it doesnít lock the screen. Sometime its fine as Iíve set the screen to turn off in 30 seconds. However, I noticed that XBMC keeps the screen on and I end up finding a dead battery the next day because the cover didnít turn off the screen.

Do I have a faulty sensor? Its not the case because I tried another case/cover and it does the same thing.

Thanks