For any talkgroup to be seen on any site, 700 or 800, you need at least two things. First, the talkgroup has to be permitted on the site. Second, there must be at least 1 radio affiliated to that site on that talkgroup.
If you are in Troop G's area monitoring G-DISP-1, and there are 50 radios affiliated to Troop G on the 800MHz Zone 1 and none on the 700 zone 21, you will not hear G on the 700 sites. As soon as 1 radio affiliates with the 700, the talkgroup gets sent to the 700 Sites in that zone.
For scanners, be sure you are programming in the P25 TGID and not the Uniden TGID#. I figure you have that but never hurts to check.
chris