Have you enabled logging and checked for errors? (See the first link in my sig). If no obvious errors, attach the actual mod (as that way we don't have to guess what you've done), see the second link in my sig
After reading your reply, I went in game to test again, I used this code for the intelligence agency, and got no result.
<Update>
<Where Type="BUILDING_INTELLIGENCE_AGENCY"/>
<Set Building_ClassesNeededInCity="BUILDINGCLASS_CONSTABLE" Building_PrereqBuildingClasses="BUILDINGCLASS_CONSTABLE" NumBuildingNeeded="-1"/>
</Update>
Also to note, in the same xml, i have
<Update>
<Where Type="BUILDING_HERMITAGE"/>
<Set PrereqTech="TECH_ACOUSTICS"/>
</Update>
<Update>
<Where Type="BUILDING_BROADCAST_TOWER"/>
<Set PrereqTech="TECH_REPLACEABLE_PARTS"/>
</Update>
and hermitage works as intended, while broadcast tower doesn't.
Could you please tell me what was the name of the mod you used <update> to change national wonder, I'd like to take a look and see if using the same code works, or if there's some problem from another part of the file that messes other things up.
If you or anyone else reading this want to, here's the link to the xml file in question:
https://www.dropbox.com/s/641qax0tr5tgs0y/UpdateBuildings.xml?dl=0