Are the section strings keyswitches working for you? Try an "F" keyswitch as your test (since it brings up the obvious change to pizzicato). If the section strings are working for you but not the solo strings then you may wish to contact us privately so that we can troubleshoot more thoroughly.
P.S. Make absolutely certain you are in the correct octave for the keyswitches.
Well, it appears that I was confused about the octave numbers because in Overture I set middle C to be C3, which doesn't coincide with GPO's mapping (is middle C *really* C5 in GPO's mapping?!), so...well, you can guess the rest.