Just because you want a discounted ticket doesn't mean they will sell you one.
It is quite possible that there is no A/Z fare published or no inventory available on the dates you are traveling or there are fare rules (min/max stay, advance purchase, etc.) which will preclude you from getting the discounted fare.
Do you have specific dates? Someone can probably look at the details and tell you why it isn't pricing as you'd like.
ETA: Looking at one of the Z fares published on TYS-GVA I see the following restrictions:
Day/Time restrictions
FROM THE UNITED STATES -
PERMITTED TUE THROUGH SAT ON EACH TRANSATLANTIC SECTOR.
TO THE UNITED STATES -
PERMITTED SUN THROUGH THU ON EACH TRANSATLANTIC SECTOR.
Advanced reservation/ticketing restrictions
RESERVATIONS FOR ALL SECTORS ARE REQUIRED AT LEAST 28 DAYS
BEFORE DEPARTURE.
WAITLIST NOT PERMITTED.
Minimum stay requirements
TRAVEL FROM LAST STOPOVER MUST COMMENCE NO EARLIER THAN
THE FIRST SUN AFTER DEPARTURE FROM FARE ORIGIN.
Are you meeting all these requirements?