Why was Constantine getting frustrated with the church?

During the reign of the Roman Emperor Constantine the Great (AD 306-337), Christianity began to transition to the dominant religion of the Roman Empire ... In 313, Constantine and Licinius issued the Edict of Milan decriminalizing Christian worship.

Centers of powers existed in Rome, in Alexandria, in Egypt and in Antioch, Syria. Language difficulties apart, disagreements on various issues of interpretation as well as corruption created fissures within the Christian religion and thus threatened the unity of the empire itself. Therefore, Costantine struggled to resolve the issues that divided the church. His first challenge was in North Africa, where rival groups were seeking his favour. The church effectively became subject to the rule of the state in that an appeal from the synod went to the state. Already existing structures within the church appeared not to matter the emperor. He put himself in a position to determine what kind of religion Christianity would be. He even declared himself a bishop.