1. Working code: Quickly, I find in your code some diferences with mine. Try out this :
topojson \
--id-property su_a3 \
-p name=name \
-p name=NAME \
-o topo/uk.json \
-- topo/subunits.json \
topo/places.json
I'haven't test it however. The topo/... path is also a difference with my code.
2. Missing: A possibility is that you lost this property upper in your workflow. The GIS file's data attribute name may have changed, etc.
3. Case sensitive: Check that the keys you call in your TOPOJSON match the keys within your GIS / Geojson file. This is case sensitive. To check within the shp file : QuantumGIS* > load the .shp file > Right click on layer > Open attribute table > There, look at the column's title.
*: or other GIS software