I've had this happen in a few locations. One I specifically remember having trouble with was in Hew's Bane, can't quite tell what it was called but it was pretty centered on the map. I tried the same things, logging off, coming back with a different character, changing zones, all kinds of stuff, nothing worked. Until, by total chance, I discovered the location - a good bit further south than the icon would have indicated. It happened in 2 or 3 other places as well and the solution was always similar. Have you tried just running around the vague area indicated by the icon without being too fixated on its exact location? Sometimes it's also that you have to stand in a very specific spot to make it work. But until now I could always somehow get the POIs to change to "discovered", even if it took awhile.