Serbian citizenship after grandparent?

My grandfather (and his parents as well) was born in a Yugoslavian town which is now Serbia. His birth certificate was issued from Serbia but according to him he was not a Serbian citizen although he didn't know whether he could have claimed it or not. He is no longer alive.

Now my mother and me would like to get the Serbian citizenship to have another passport. May we get the Serbian citizenship based on the fact that my grandfather and his entire family was born in a Yugoslavian town which is now Serbia? He was living there till he was ~12 years old then he left with his family. I have his birth certificate issued by Serbia. Was he a citizen of Serbia or could he have claimed citizenship? If he wasn't but he could have claimed it, may we claim him to become a citizen in retrospect so we (my mother and me) could get the citizenship after him? I have sent several emails to the local Serbian embassy and I have talked with them over the phone and they wasn't sure. Sometimes they say yes sometimes they don't know. I have sen't them a copy of my grandfather's birth certificate and they promised me several times to check the matter but they haven't been doing anything for months now so I think I'll travel to Belgrade personally if someone can confirm me that we have the right to the Serbian citizenship.

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