If mapview has wrap around enabled, then once you tap on a graphic, a callout sometimes shows on the other instance of the graphic - the instance that can be revealed by scrolling the map around.
For example I have a graphic placed in north america. I zoom out all the way and tap on the graphic. At first look it appears that callout did not work - there is no callout where I tapped. But if you scroll the map, it will wrap around and north america becomes visible again - this instance shows your popup.
