As others have noted, you're most likely missing an identification rule or mapping the wrong data into a field (so as not to cause a match). You can enable debugVerbose logging and check out what's going on in the process to confirm.
Properties for Identification and Reconciliation (servicenow.com)
In your script, it would also be a good idea to set a Discovery Source to make things easier to understand in the logs:
cmdbUtilIRE.setDiscoverySource([source name]);
If you haven't already, you'll need to register the Discovery Source first, though. You can do that in a fix script:
var sourceName = “[source name]”,
dsUtil = new global.CMDBDataSourceUtil();
dsUtil.addDataSource(sourceName);
Looking at your initial post, it should be a given, but we ought to confirm...your Import Set's target table is in the CMDB, correct?
