A lot of stuff gets down to timing. Is it the right time?
God asks His kids to wait on Him, wait on His timing. If we do according to Isaiah 40:31 we will gain new strength. I think that new strength is God's strength. We will be operating in His power because we waited till He said yes, go ahead.
If we wait on God it goes a lot better. He has a plan. He sees everyone and everything. Often, when He works in our lives (and actually probably always) when He answers your prayer, when He provides, when the miracle comes (etc, etc, etc) it isn't just about you. He has other people in place. Maybe someone needed to see that miracle. Maybe someone else needed to see that God actually will provide for His children. Or maybe when He answers that prayer you had to be in a different place emotionally or even physically.
I'm waiting for God to heal my son Hawk. Hawk wants to walk. Right now Hawk can't walk or talk or sit. He can roll around really well and he is very flexible. He can do the lotus position backwards (meaning as he lays on his stomach he does that lotus thing behind his back). But he can't do any self-care tasks as of yet.
I don't know why God hasn't healed Hawk yet. It isn't because we don't have faith. Maybe it is because a certain someone needs to see it happen and that person isn't around yet. Maybe it's because God wants Hawk older so it is an even "Bigger" miracle in people's eyes. That guy in Acts had been unable to walk since birth (the guy that sat by the Gate Beautiful). He could have been healed prior to when he was but maybe God wanted all the people who passed him for years to see that God does indeed perform miracles. No one is ever a lost cause.