Hi,
After installation tooling is run to upgrade indexes to the latest format.
If the process has stuck, you will need to check system logs to see if there is any indication of what is wrong.
In admin UI in the top bar, click "System" and navigate to "View system logs". In the list in "Web logs" section look for "push.log".
When upgrading Funnelback you don't need to do it version by version. I would recommend upgrading it straight to v15.24 and just following release notes for each previous version to apply any manual changes.
Please don't forget to apply the latest patch as well to your final version.
cheers,