zooming in may be the difference, do a test, and mark a waypoint zoomed all the way in, and mark another spot when zoomed out some, maybe 50 or 100' and see if that makes a difference. I use a garmin HH gps, and have waypoints/structure marked/saved, and if you miss the spot by just a little bit, the HH will not beep. I can see the structure on the FF but I may be off the "mark" a bit.