The reason this question hasn’t been answered is that it is too vague and does not describe an actual problem.
To the original poster @josuesantana: Your follow up is just a rewording of your first post and doesn’t clarify anything.
To the second poster @mhamudul_hk I will elaborate –
If Loco’s sync removes 500 strings, then it could be that those 500 strings are not actually in the theme. Please verify if that’s the case.
It could also be that the source code is not in a format that is extractable. Please verify that too.
If you don’t how to check these things, then at least provide a link to the theme you’re asking for help with.
If the POT file shipped with the theme is not up to date with the source code, you should report it as an issue with the author. Loco cannot account for incorrect POT files in themes and there is no guarantee that string extraction will be performed exactly as the author intended.
If you wish to override the author’s template (or generate one when there wasn’t one) then you should know something about how string extraction works. See working with templates