AI traffic service not available VFR

A recent occurrence ... if I do not do a flight plan, I am normally able to request taxi clearance etc VFR - but not anymore. I can still bring up the ATC window but pressing any of the keys for REQUEST TAXI - EAST DEP etc' does nothing. This used to work just fine, but can't work out why it doesn't now. If I do an IFR flight plan the ATC still sorks fine.I noticed also that the airport (which is normally ATC equipped) doesn't come up in the CHOOSE AIRPORT list either - if I depart and then turn around and come back after a while.I'm baffled - any ideas please???