All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
There is no doubt that your father is entitled to inherit from his father as long as your grandfather had passed away first. Your deceased father’s share in your grandfather’s estate is to be transferred to the father’s heirs. In fact, your paternal uncle is not allowed to deny you your Sharee'ah-prescribed share in the grandfather’s inheritance. The claim that you are excluded from the heirs entitled to inheritance is totally groundless. Indeed, you are entitled to your father’s share in the grandfather’s estate and you are not excluded by any other heirs. Instead, your paternal uncle is prevented from inheriting from your father, since a brother is entirely excluded by the presence of the deceased's descendants. It should be noted that the share to which you are entitled does not represent your share in your grandfather's estate. Rather, it is your father’s share of his father's estate.
As for your grandmother, your father is not entitled to inherit from her because he passed away before her and you are not entitled to inherit from her as well because you are fully excluded by the presence of her direct children.
To conclude, you are entitled to your father’s share in the grandfather’s estate. The fact that your father's share was not officially registered does not justify denying you your share in the grandfather’s estate and depriving you of your right. You are entitled to your father’s share in the grandfather’s estate whether your father's share was registered or not. Denying you your rightful share constitutes injustice and a form of devouring people’s wealth unlawfully. Whoever does this is exposed to the punishment promised for those who devour people’s wealth unlawfully.
You stated that your father had owned land and other properties and some of them were registered under the grandfather's or grandmother’s name and others under the name of your father and his siblings (co-ownership). Such a claim requires evidence to be submitted to the court.
Finally, we advise you to refer your dispute to a Sharee'ah-court if any. The fact that you do not possess official papers and documents is not a problem as long as you have witnesses to testify that your father owned such land and properties and that he had passed away after your grandfather and that you have received nothing of your father’s share (in the grandfather’s estate). If you know that the court will not grant you your rights, you are advised to seek the mediation of wise and pious people who are believed to have an influence on your paternal uncle. If he remains firm on his stance, you can invoke Allaah to enable you to take your rights. Even if you are denied your rights in this worldly life, you shall, certainly, get them in the Hereafter.
Allaah Knows best.