How, exactly, did you install it?
VirtualBox and LibreOffice are a little exceptional in how they are handled in PCLOS (one due to licensing, the other due to size).
When you update Synaptic, it just updates a script that does the installation / upgrading for you. In the case of VirtualBox, run the "getvirtualbox" script (under More Applications > Emulators > Get Oracle Virtualbox). This script will also handle getting and installing the "Extension Pack", which is what allows USB, amongst other things.
The "tricky" part comes when you update...keep an eye open for updates to the script(s), and after it all done, you have to (re-)run the scripts to get the actual updated program.