nope .. you've just proclaimed that God is inconsistent and cannot be trusted to be faithful to his word .
think about it .where does the lord JEsus say all things are possible to those who believe .."sometimes". ?
I've done nothing of the sort.
The fact God doesn't always work in a formulaic way that we can define doesn't mean he cannot be trusted. If God worked in accordance with our wishes he wouldn't be much of a god, he would be a puppet on a string with us pulling the strings. Frankly any god that does what we tell it to do, when we tell it to do it, isn't worthy of our worship because it is clearly a lesser being than we are.
It's easy enough to pull a verse here or there to try and prove a point but you still have to explain why people all over the world are not healed. If it's always God's will to heal why are there still sick people? Is God less powerful than we are? Can our own uncertainty block the power of the Almighty? Surely not, or we're back to the issue of us being stronger than God again.
I don't see the difference between some being healthy and others being sick as any different to the difference between some being rich and others being poor, or some being strong and others being weak, or some being intelligent and others being unintelligent. Why does God apparently bless some and not others? We might start an answer to that question by considering what "bless" actually means.
I know a lady in her 60s who is a wheelchair user. She hasn't been physically right since she was born. She can walk short distances, leaning heavily on crutches and moving very slowly and painfully. Years of doing that before she started using a wheelchair all but permanently shifted the load of moving around from her legs to her arms, and trashed her shoulders in the process. She's a very Godly woman, and in her words God can use her in her condition in ways that just wouldn't work if she were fully able-bodied. So taking one viewpoint she's been waiting over half a century for healing that never came, but from another viewpoint she's exactly where God wants her to be.
Jesus also told us to take up our cross and follow him. He never said it would be easy. He never said that if we believed enough our crosses would be taken from us and we could take a gentle stroll down a nice easy path.