Not possible to add new instrument

I imported a orchestral composition via MusicXML from Logic Pro. After that it is not possible to add a new instrument. The new instrument is invisible in the score and also in ‘play’. At the attached screenshot I tried to add a second tenor trombone.
What I’m doing wrong?

Is the new instrument assigned to both the flow and the layout? Your screenshot makes it impossible to see.


That shows that it’s assigned to the layout, but what about the flow?

Sorry, I attached a new foto!

Sorry, problem solved! It was the flow!
What a stupid mistake!