New instruments don't appear.

I’m adding some Players to an existing document. They don’t show up in ANY Layout: not even their own. They are ticked, and when I select them in Setup, the Layouts are highlighted on the other side.

This is happening on two documents I’m working on today, but other documents behave normally. I can keep adding instruments, and none of them shows up in the Full Score, and their Part layout is not even a page.

What’s going on?

Have you added them to any flows?

Ah, there we are. Why does that happen automatically in some documents, but not in others?

I can’t say I’ve ever paid any attention to it, to be honest. It’s six of one and half a dozen of the other for me - sometimes I want extra players in every flow and sometimes I’m cursing Dorico for messing up the nice layout work I’ve already done!