A trick that has really helped me with filling in the gaps is this: go by the nephews and nieces names.
You see, Scotland used to have a strict naming tradition, explained in this blog post here
As you can see, children were often named after their aunts and uncles, so if you have one ancestor, and are trying to see if they had siblings, have a look at what the existing brothers and sisters named their children. This works best if you have two or more siblings in the same family. Cross off the names of the parents, maternal and paternal grandparents, and names that were used on the “in-laws” side. Review what you have left, then line this up your search results.
Beannachd leat! Good luck!