Why did Ananias fail to fulfill his promise?

Ananias failed to fulfill his promise because he lied to the Holy Spirit and kept back part of the money he had promised to give to the church. This act of deceit and dishonesty ultimately led to his downfall and punishment by God.

Ananias did not fail to fulfill his promise. He and his wife Sapphira sold a piece of property and brought part of the proceeds to the apostles, claiming that it was the full amount. When Peter confronted them with their lie, they both fell dead.