In the following scenario, MapTips does not display after the primary display field is changed:
The map document (.mxd) is originally created using uncompressed file geodatabase feature classes (fGDB) with MapTips enabled for a primary display field (example: NAME) set in the layer properties: Layer Properties > Display > Show MapTips checked.
If the original fGDB is compressed after the map document is created, MapTips does not display if a new primary display field is set because the compression utility for fGDB uses Smart Data Compression (SDC) format.
As stated in the ArcMap Context Help: 'MapTips are not supported for SDC data'.
When the map is re-opened, MapTips displays as the original primary display field because it is saved on the map document level. However, as soon as the primary display field changes, MapTips are no longer accessible. The MapTips check box is grayed out for the reason explained above.