hi,
There are some issues might cause ...
1) Please Coordinate system...is your data is projected ?
2) Overview map all settings are correct ?
there are some solutions may help you...
1) on Page_Load
Refresh your overview map
OverviewMap.Refresh();
2) add on page_load or OnPreRender
OverviewMap1.Extent = Map1.Extent;
Above i have tested perfectly working for me...Please check any overview map configuration issues.
Thanks and Regards,
Venkat Tammineni