My original Smooth PEI sheet, with came with my printer 2 years ago, seems to have given up the adhesion ghost even after a couple of thorough cleanings. Fortunately I have a new one on standby.
But for educational purposes I'm going to try the "last resort" of cleaning it with acetone today. It was stored in the garage which is at 44ºF and I brought it in to warm up a bit first.