Jesus gave us His authority to use His name. His name is the greatest power in the universe. We can use it to speak to the enemy and declare victory over any situation that we struggle with. But sometimes we need to pray and fast.
I don’t know why, but there are some situations that we don’t just win outright. A man brought his epileptic son to Jesus when the disciples couldn’t help him. Jesus said that this kind does not come out without prayer and fasting. (Mat 17:15-21)
So we know that we will run into situations that require more than faith, declaration, and prayer. Prayer and fasting together are a unique weapon against especially strong enemy resistance. I don’t understand how it works, but it does.
It may be that God will tell you when it’s time for prayer and fasting, but I would suggest that if your normal prayer does not work, then it’s time to pull out the big guns. Be sensitive to fasting. It can be done in any number of ways, but it needs to be something that you miss terribly by not having. Ask the Holy Spirit to help you know how long and what you should do.
Your sacrifice builds a power against the enemy that nothing else does. Prayer and fasting will breakthrough the hard places.
Another post on prayer: How Do I Pray?