One possibility is that your sensor version is higher than that your IDS-MC recognizes. The following procedure was handed down to me. I feel it would be a good idea to cross-check the procedure before opting for it:
1) Download the IDS-K9-sp-4.1-3-S61.zip update from the software center
2) The step by step update process is present in the accompnying readme file
3) Open Security Monitor and execute the zip file: Admin > System Configuration > Update Network IDS Signatures
You can then apply subsequent Service Packs and Signature Updates and then discover the sensor. So long as the signature revision is later than that of the sensor, things should work fine.