We have a 3rd party intranet which uses SharePoint Online as it's back end, this is where our documents are stored. We have been using a library in a cassic SharePoint site to store documents displayed on the intranet, however, we are now moving everything to Teams and as such want to be able to do the same things with a library in a Teams site.
We have run into an issue where documents on some Teams sites are not searchable. I have a result source configured to look at the old SharePoint sites so updated this to point at the new Teams sites but the query returns nothing. What is odd, however, is that when pointed at other Teams sites it brings back items just fine. I can't see any significant difference between these sites, they are both private and were created by first creating a Team from Teams itself.
The only difference I've noticed is that under settings the site that does return results has the "Users and Permissions" section whereas the site which doesn't does not.
How might I go about diagnosing what the problem is here?