Can't add instruments to condensing group

Dorico isn’t automatically putting my trumpets or trombones into a condensing group.

and if I try to add them to a condensing group they don’t appear in the list of instruments.

How do I get them to condense?

Your trumpets are section players. Section players don’t condense. You need to use Solo players.

Ah! Of course. Thanks.

Section players don’t condense for now, but in the fullness of time they will.