Were your parents married to each other when you were born?
This is the question that decides most claims through a Thai father. Under Thai law a child born to parents who were not married to each other is the legitimate child of the mother alone, so a Thai father cannot pass nationality automatically unless he was married to the mother or has since legitimated the child. Legitimation means a registration at a Thai district office, or a court order - not simply being named on a birth certificate.