The power switch uses a little 'hairpin' spring, so there must be some kind of restriction (dirt, hair, ?) that has gotten under the switch. There is a video clip on this forum that shows a disassembled K3. The slide simply passes over a microswitch to wake the unit or power up -- extremely simple and should be foolproof except for dirt impeding the path.
https://www.mobileread.com/forums/showthread.php?t=97266