The accelerometers are clearly a bit flakey on these tablets. Mind had similar issues as yours and the thump method worked for me. Since then, it won't spin the screen when I rotate it so I had to install an app to turn the screen for me. Mine is fucked these days anyway. I've got a couple of big dents in it and even after a recent factory reset, its still incredibly slow at even the most basic tasks. I've had a lot of use out of it though so I'm not bothered and there is allegedly a new one coming out next year so I suspect I'll just upgrade.