On my skills test I was given a diversion to Shotteswell, just north west of Banbury. Completely invisible. I eventually saw the tiny windsock after flying around in circles for a bit. The examiner was greatly amused.
Since then I have, like everyone else, failed to see Fenland (like Brig a Doon, it only appears once every hundred years) and Lydd. I also recall one fine day with an instructor just before my test, after a wizz-bang sortie involving instrument flying, a practice pan, etc etc, when neither of us could find Stapleford. We think that it had gone to a parellel dimension (some call this Essex) but then came back again.
I can usually see White Waltham, except when coming back from the west, when I am too busy crashing into the non existent mast near Reading.