search results not showing all site content

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Hi, we're having a problem with the search function on our site right now. When you do a search, some pages' content don't show up in the results page even though they are included in the page content. I have noticed that custom block content don't show up in some searches.
So I re-formatted some custom blocks so they can be added using the built-in content block, and that part showed up in the search results.

I have tried a lot of things as suggested by other users in the forum. stuff that worked for them, but sadly not in my case.
examples are:
- cleared cache
- re index the site
- edited the search_index.php file by adding this line of code; sometimes more than 2 lines for different areas on the site
$is->addSearchableArea('Teacher Led FAQ');

- adding the
function getSearchableContent()
to the custom block's controller.


I've been at this the whole day, and if anyone can point me to the right direction that would be great!
I'm now confused as to how the search function actually works in C5; like which content does it include or not?

The site is currently running on 5.6.1, and if anyone needs the link I can gladly provide it here. :)