Married a long time but no children: whose fault is it?

No one should be blamed; children are a gift from Allah, and fertility problems can occur in men or women.
Allah says that He grants daughters, sons, or both to whom He wills, and makes barren whom He wills. The practical step is a joint fertility check-up at a clinic, because male causes are also common. Keep making du'a, including the du'a of Prophet Zakariya. Avoid blaming the wife or pressure from family. If a wish for polygyny arises for the sake of children, it must be discussed honestly with the wife, after joint treatment has been tried, and with care for the wife's feelings and rights. See Fertility & IVF and Counselling.