Objects in full score not appearing in part

I have a solo instrument piece which I have been entering/editing in full score (the music is for transposing instrument which shows up in the “part” layer). However, some notation objects are not being carried over from the full score to the part, namely the trill line. I have this set in full score to show trill line. Is there a way to force this to carry over to the part?
Many thanks,

I suppose a second question would be - if the score is just for a solo instrument, do you write/edit it in “full score” or “part”?

In answer to your first question: as of Dorico 3.5 there is a “Set local properties globally or locally” switch at the top right of the properties panel. Set it to global if you want property changes to carry through to all layouts.
As in previous versions you can still select an object/passage/all and go Edit > Propagate Properties. Note that Dorico will only propagate properties that apply in the mode you’re in - this is a smaller range of properties in Write mode than in Engrave mode.

For solo pieces, it really makes no difference which layout you work in as long as you stick with that one layout. No harm will come to your project if you delete the spare layout.

Which layout your choose may depend on which Master Page Set (score or part) comes closer to the layout you want.
Of course, you can always reassign the Master Pages later in Layout Options if you change your mind.

For the trill line specifically - if you’re editing lots of individual trills to show a trill line, you might instead find it easier to change the default option for this in Engrave>Engraving Options>Ornaments. I think by default, trill lines only appear on tied notes, but I also like to show them on single-notehead-notes, so I set this engraving option accordingly.

In Dorico, there are engraving options for an awful lot of things - if you find yourself changing a property for the same type of item lots of times, do pop into Engraving Options as chances are, there’s a default setting for that.