This is happening more and more for me of late, and it has me quite concerned, as last night there was even a semi-random collection of instruments that were available once launch succeeded the first time, so I desperately brought up License Control Manager to make sure my Synchrosoft USB Key wasn't damaged and saw all the licenses.
I do find though, that multiple restarts are sometimes required to bootstrap VI (I can't use VE as it crashes even on simple stuff). Also, it usually helps to launch VI in standalone mode outside your DAW before using it in your DAW. Also check to make sure audio and MIDI settings are correct, as this sometimes screws up usage inside your DAW.
Of course I have an ancient computer, which will be replaced this summer once I have time to devote to the switchover. I am hoping that the behaviour is strictly related to having an 800 MHz G4 iMac and being so far behind in recommended minimum specs. But VSL also noted in another reply (to me, but on the forum) that USB keys can wear out.