1. You must read the Bible and see that the weakness is a part of Christian life.
"That is why, for Christ's sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong." 2Cor 12:10
2. You must also see that not everybody was healed in the Bible. And not because of the lack of faith.
3. You must also see that for example Epafroditus was ill/got better but Paul does not describe it like some kind of "automatical power every Christian has", but like a mercy of God
If you simply add 1 + 1 together and accept all the Scripture, you cannot believe in the WoF teaching.
"But I think it is necessary to send back to you Epaphroditus, my brother...For he longs for all of you and is distressed because you heard he was ill.
Indeed he was ill, and almost died. But God had mercy on him, and not on him only but also on me, to spare me sorrow upon sorrow."
Php 2:25