Importing software via the Software Library method happens very slowly. In some cases or with larger packages the import will timeout. The progress bar's progress may also slow down over time.
It is possible the services or IIS got in a bad state and caused the issue.
If this does not fix the issue, please contact Symantec Support. Collect server logs from %programdata%\Symantec\SMP\Logs to give to Support.
If preferred, the package source can be defined as a location other than Software Library. When software is sourced from the library, the NS creates a copy of the software files and places them in the library. If another source is specified, the package will not be copied over to the Software Library. Doing this has solved issues importing large software packages for some customers.