Thanks for the reply. Unfortunately I had to completely reinstall the laptop where this occurred and when installing it again it didn’t happen again…
From what I remember is that I initially had everything selected. But right after installing I opened up the ‘Manage’ tap and de-selected one folder (a backup folder that I didn’t need to have locally). It already had some stuff in the queue so perhaps it then stopped adding folder to the queue(?).
I started fresh (new laptop). The check-boxes on the selective sync were green (I think, i’m a bit colour blind) and I had to click and open the folder to get it to sync. It did that only for that specific folder and not any child folders.
Newly added files were added correctly, I was just missing the already existing ones.