Theophylact of Ohrid

If he believes, let him ask, but if he does not believe, let him not even ask, for he will receive nothing of what he requests. The doubter is also the one who asks with arrogance. But it is evident that whoever asks with doubt is an offender, for if you are not confident that God will fulfill your request, then do not approach with a petition at all, lest through senseless double-mindedness you become an accuser of Him Who can do all things. Therefore one must implore deliverance from such a shameful malady.

Commentary on James ↗

Oecumenius

But let him ask in faith, with no doubting. For he who wavers is like a wave of the sea, driven by the winds and swept away by force. "he who wavers." For if he trusts, let him ask; but if he wavers, let him not even ask, because he who does not trust that he will receive, will not obtain. "like a wave of the sea." For he who wavers, doubtful about his requests, after having endured for a little while, immediately withdraws. …

Commentary on James ↗

Bede

For he who doubts is like a wave of the sea, driven and tossed by the wind. He who, with a conscience biting him for sin, doubts about receiving heavenly rewards, easily abandons the state of faith at the impulse of temptations, in which he seemed to serve God in peace, and at the whim of the invisible enemy, as if by a gust of wind, is carried away through various errors of vices.

Commentary on the Catholic Epistles ↗

Bonaventure

"If any of you is wanting in wisdom, let him ask it of God, who gives abundantly to all men, and does not reproach; and it will be given to him. But let him ask with faith, without hesitation." For one who hesitates is not disciplined. This, then is the door. Such a desire extinguishes all others, and makes man to be lifted up from the world.

Collations on the Hexaemeron, Collation 2 ↗