Undo Retention Question

 I have my undo retention set to 14 days and my undo tablespace is allocated 22 gigs but only 3 gigs of it is being used.  When I try to do a flashback query,  the most I am able to go back is 7 days.  Why can’t I go back 14 days? Why is it not using all the undo tablespace for my retention?  I originally had my undo retention turn on but my undo tablespace was almost full so to prevent future transaction from failing I turned off undo retention.