If you are unable to view the devices in ZENworks Control Center:
On the Primary Server, check the files in the <Installed Path>ZENworks\work\collection\inventory zipped folder to see if the loader is running.
If the loader is running, verify whether any PRU deploy or KBMerge process is also running as this stops the processing of WIF files until the process completes.
If none of these are running, check the loader-messages.log to see if you find any log message stating that the PRU version is not in synchronization with the LMPRU version. Apply the PRU as the LMPRU to ensure that the PRU versions are not in synchronization.