It’s often said in our times, “Pray for something once and let it go. Trust that God heard you and that he will do what’s best for you. To keep on praying for the same thing is a lack of faith—you’ve got to just trust God.” It sounds reasonable. But it’s not what Jesus said.
“Then Jesus told his disciples a parable to show them that they should always pray and not give up. He said: "In a certain town there was a judge who neither feared God nor cared about men. And there was a widow in that town who kept coming to him with the plea, `Grant me justice against my adversary.' "For some time he refused. But finally he said to himself, `Even though I don't fear God or care about men, yet because this widow keeps bothering me, I will see that she gets justice, so that she won't eventually wear me out with her coming!' "And the Lord said, "Listen to what the unjust judge says. And will not God bring about justice for his chosen ones, who cry out to him day and night? Will he keep putting them off? I tell you, he will see that they get justice, and quickly.” Luke 18:1 (emphasis added)
Jesus told us (as his disciples) to keep on praying for the same thing (within Biblical parameters of course) and to not give up until we get it. He wants us to be so engaged with him, so involved in asking him for things that when we get them, there is no doubt that our Father did it!