This is a known issue when using an unsupported browser version such as IE 9 on NNMi 9.1. If you wish to use IE 9 then you will need to updgrade to NNMi 9.2. Here is the section from the NNMi 9.1 Support Matrix:
o Microsoft Internet Explorer version 9 Internet Explorer 9 (RC) has many known issues, including non-functioning menus, no map display, and table columns cannot be selected
o Microsoft Internet Explorer version 8 when running in Compatibility View mode
Be sure to disable Compatibility View in Internet Explorer 8 using Tools → Compatibility View Settings (clear all check boxes).
o Microsoft Internet Explorer version 7
Internet Explorer 7 (or Internet Explorer 8 in Compatibility View mode) has many known issues, including many layout issues, no ability to close the Help → System Information window, and inability to change selections for CSV export.
o Microsoft Internet Explorer version 6
o Apple Safari (all versions)
o Opera (all versions)
o Google™Chrome (all versions)
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