I think the problem stems from a recent update. For some reason my taskbar was not auto hiding each time I booted my PC and for a few days I gave it the 3 finger salute and simply restarted Windows Explorer. I've now dropped a .bat file into my startup folder to do it for me. It works but I hate not knowing what the problem was or what the 'proper' solution is.
Has anybody else had this recently (I know it's been happening for years). I'm running Win 10 home edition Ver.1709 Build 16299.64.