Hi Simon,
welcome to the forum.
If you are copying and pasting whole bars, you can use the System Track. Select the bar, make a tick at the right of it and then copy/paste. This copies everything of that bar - throughout all systems:
You can also go the conventional way and select something in the bar, and then copy and paste. If tuplets/triplets are involved, make sure you also select the tuplet signpost. If you can’t see the signpost, make them visible (View Menu).
Hope this helps.