Hi,
Sure!
First, make sure the correct post types are selected on the search sources, often times it’s completely missed. Then, if you are planning to redirect the results to the results page, check if the results page override was not disabled by accident, it should be turned ON by default.
Sometimes inclusions or exclusions can cause similar issues – make sure nothing is excluded nor included in the first place, so it does not conflict with the potential results set.
And finally, items which may not be tagged or categorized are shown by default. If you prefer not displaying those items, you can turn that off here.