How to leak proof roof?
You have either, too flat of a pitch in your roof or installed your roof tiles incorrectly. Just accept it and move on.

It's going to have to be redone. You would be wisest to tear it all out, then get someone who knows how to do it correctly in to do it. If you half-ass it now, it will just come back later, usually after causing structural damage for years.

Sorry, I know the truth hurts, but it's better you know now.