The "Last Updated by Inventory Server" field does not reflect that the scan was successful, but only that the scan was received. It could have been rejected for various reasons.
The best field to check is "Last Software Scan Date", this is the last "Full" inventory scan.
My guess is that your clients are not sending or the server is not processing a complete inventory scan.
I would further guess that the vulscan is not being successfully processed by the Core, or that the client is not behaving as it should