Bottom line: Fervency and perseverance are required to wrestle in prayer; although our prayers may be immediately heard, the answers to our prayers may be delayed.
The moment Daniel began praying, God heard his prayer and sent an answer. However, the angel that was bringing the answer was delayed by spiritual warfare with a demonic being (the Prince of Persia).
If we could look behind the scenes, we would see angels and demons fighting battles. And in some way our prayers are connected to the battle. As Paul wrote, we don't wrestle against flesh and blood. We wrestle against demonic forces. Warfare praying requires exertion. We have to intentionally be fervent and persevering in our prayers. Let's hold fast. God's answer is on the way.