That depends on a lot of factors, but mostly on the engine of the game and the version of Translator++.
(For example old VX versions are bound to make problems and trying to translate Wolf RPG is generally a waste of time.)
This thread is pretty outdated, you can find newer Translator++ versions
here.
Another common thing can be the game structure in general. Sometimes devs move things around for seemingly no reason and you have to move them back into the standard folders so Translator++ parses it. (But then back for the game to actually function.)
Also sometimes a big chunk of the dialogue is actually inside the, troops, scripts, and plugins, not where you would expect it.
I would strongly recommend not using the inject feature and instead exporting the project because that will show you exactly which files were actually modified.
Edit: Should the game data still be encrypted in a rgss3a (Ace) rgss2a (VX) or rgssad (Super old VX) you can decrypt it to the individual files by using
All Decrypter. That would make it easier to find stuff.