What version of CUOM are you running? I would recommend installing CUOM 2.0.3 which is the latest maintenance image of CUOM and has many bug fixes.
Please do the following to gather more information on why it is not going to monitored state.
1) Go to Devices > Device Management > Modify/Delete devices
2) Select your device.
3) If you scroll down there will be information on the "Error Code" and "Error Message".
4) Based on the error code and error message use the following section of the user guide to troubleshoot.
http://www.cisco.com/en/US/docs/net_mgmt/cisco_unified_operations_manager/2.0.3/user/guide/useDevMg.html#wp1232391