not displaying transposed/concert pitch accidentals correctly

I have imported from Finale a project for orchestra where the horns in F for some reason in without the corresponding key accidentals, both in concert and transposed view. But pitches are displayed correctly in both modes. Workaround is to change instrument, but is there a setting i have missed? in Layout options the transpose box is checked.

Whether or not a horn shows a key signature is indeed a property of the instrument: you can use Change Instrument in Setup mode to change to a horn instrument that shows a key signature if you want key signatures to appear.