Ah ha! Thanks for PCF. I managed to figure it out. The valves without tag annotations are identified as being part of a vent/drain ... and the vent/drain theme in the iso styles by default have no tag annotations for valves. The Annotation settings dialog in Plant 3D only controls the 'Default' theme. This is unlike the Dimension settings dialog where you can change dimension settings for all themes (Default, Small bore, Vent/Drain, etc.)
To get tag annotations in the vent/drain theme, you will need to go into IsoConfig.xml. Find the Themes/VentDrainPipingTheme/Annotations/AnnotationSchemes node in the xml. Under that node, add the following two lines ...
<ComponentScheme Name="Tag" AnnotationStyle="WipeCircle" Grouping="true" Tag="XX" Alignment="FlatHorizontal" LeaderStyle="AsNeeded" Filter="Valve" Fields="TAG" />
<ComponentScheme Name="InstrumentTag" AnnotationStyle="WipeCircle" Tag="XX" Alignment="FlatHorizontal" LeaderStyle="Always" Filter="Instrument" Fields="TAG" />
You'll find these two lines in the 'Default' theme also (so essentially you're duplicating these in the vent/drain theme).
This should start giving you tag annotations for valves and instruments that are part of vent/drain piping as well. Hope this helps 🙂
-Karthik