When you are using unlinked GPS and radar to find an isolated buoy, enter the mark as a waypoint, check that it is within the range you have set on the radar, then note range and bearing on the GPS. Next, translate the compass bearing into a bearing relative to your heading. Now set the electronic bearing line to this and the variable range marker ring to the distance as supplied by GPS. The buoy should appear as a blip on the intersection, or very close to it. This simple technique is most useful in poor visibility when it is hard to decide which of a number of targets is your buoy.
Once isolated, double-check it as you approach. You are then unlikely to be disappointed.