For that person must not suppose that he will receive anything from the Lord;
According to verse 7, does the one who doubts in verse 6 receive wisdom if he asks for it? To say no is to disprove the premise of verse 6, that God gives generously to those who ask, giving without reproach. Does God give without reproach even to the doubter? The one in the mindset of doubt will say no, the one in the mindset of faith says yes, that God upholds his promises and I believe it.
Turn the coin over. What if this isn’t a God problem. What if the problem in receiving wisdom lies within the fallen nature of man? Now that seems more plausible. God, being perfect, fulfills his promises while man, being flawed and needing to renew his mind, may not see that he has received the promise due to the obscurity he has in the mind as a result of the fallen nature.
We like to blame shift, but on this one, I see a man problem, not a God problem. This is why James says that the doubter should not suppose (think, believe, imagine) that he has received, not just wisdom, but anything from God. This person who is doubting needs a reset of the mind. He needs to stop operating out of two psychosis. This man needs to operate fully from the new self, which is driven by the knowledge of Christ in him.
Therefore when one doubts, one needs to stop thinking all together about the relation between God and his blessings and refocus on the elementary principle of the Gospel, that he needs to repent, i.e. change his mind.