I had the same thing happen a cople times with mine. I found that I still had some music streaming to it in the background even though I had moved off the browser to another screen. Once I stopped the stream it slept.

Another time I found that clicking "Room Off" on the home page got it to sleep.

Finally another time I could find nothing so I did a hard reset by flipping the power switch with a stylus on the back of the unit.

That took care of it !