I had the same problem with Patch Manager. I even contacted Support for it! We found that there was a "minor" glitch on my scheduled tasks for Inventory and Discovery... Patch Manager originally was from another company - EminentWare - that Solarwinds purchased and the original idea behind the discovery and inventory tasks was a mess. Those tasks affect the way computers are displayed or not under "Managed Computers". Recheck you Inventory task and the operations you've selected for it. If needed, disable your current scheduled task and try to build a new one; you'll see after a few days your systems being correctly discovered and inventoried.
Send me a message if you need further help.
Thanks for your response. I've recreated the scheduled inventories as per the following kb:
It seems to have done the trick.