Luke 18 begins with this verse. Then Jesus told them a parable about their need to pray always and not to lose heart. 
All of us have been discouraged in our prayer. The reasons for that are fairly obvious. Mary Queen of Scotts reportedly said she feared the prayers of John Knox more than all the armies in Europe. Well our enemy, the enemy of God, is terrified of your prayers. And he puts great energy into his efforts to discourage and distract you from praying.
But we should know that the greatest miracles in prayer come about in those areas where answers seem stubbornly impossible. Keep praying. Pray again and again. Pray more and more. We are going to see God do mighty things in our day.