Unable To Add Apps to Mobile Device

I've got MI installed and running Netweaver 2004s SR1 on an app-server. I used to be able to add applications to a device via the Mobile Administrator (log onto NW then System Mgmt > Administration > Mobile Infrastructure > Device Maintenance). Now, however, when I select a device and click on the Mobile Components tab at the bottom I don't see a Add/Remove button. Does anyone have any ideas why this button would disappear? (Note: it's not 'grayed' out; it simply doesn't appear.)