Sometimes after upgrading to 8.0 HF3 all alerts stop working and the Monitor Plug-in Upgrade policy target never decreases.
Sometimes the 8.0 HF3 upgrade gives the Computers requiring Monitor Plug-in upgrade filter the wrong version of the Monitor Plug-in: 8.0.3108.0, when the correct version is 8.0.2518.0. To check this go to Manage>Filters>Monitor Filters and look at the SQL query inside the Computers requiring Monitor Plug-in upgrade filter. If the query shows 8.0.3108.0 then Monitor Solution will consider all the plug-ins as out of date.
Import the correct version of the Computers requiring Monitor Plug-in upgrade filter by downloading the file attached to this article and then importing it into the Monitor Filters folder by right-clicking on the folder and selecting Import. Browse to the downloaded file and click OK. Once the filter has been imported make sure that the SQL query now shows the correct version of 8.0.2518.0. Monitor Solution should return to functionality once a resource membership update runs and all the clients check in for new configurations.