The display of incoming GeoEvents pauses while a modal dialog box is open.
The GeoEvents are cached while the modal dialog box is open. When the modal dialog box is closed, the display will attempt to catch up with the current GeoEvents by starting at the last GeoEvent that was obtained before the dialog box was opened and proceeds from there.
Do not open modal dialog boxes while ArcView Tracking Analyst has incoming data from a feed.
This is especially problematic if the modal dialog box is opened as part of an action script.