I am assuming a lot of things here, your patches are up to date and they are the same level as your primary poller, the upgrade went fine and all services are up and running except for the status of additional poller (down) on solarwinds polling engine page.
Is this the issue you are facing, i am as well assuming its polling the devices properly except for the fact that it shows down on polling engine page.
Please correct me if I am wrong
Managed to get this sorted. Had to uninstall the software, deactivate the license, add a new license into the license manager on the main poller and then reinstall the additional poller again - in all it took about 4hrs of trouble shooting but got there in the end