How late in the morning did the birds start trading off the water to feed? My experience with late winter geese that have been hunted is that they pretty much know the difference between live birds and decoys when the sun is (relatively, since we are not far past the Winter Solstice) high. If the birds are sitting on the water well into the morning because of the cold weather, they are moving out to feed when they can see very well. If you have not seen birds actually feeding in that field, that has a major influence of how willingly they will work, particularly into a standing crop, since the decoys are partially obscured it. Do your field blinds match the wheat color well? Have you rubbed them down well with mud to kill glare on the blind fabric? Are you flagging? Are you allowed to use motion decoys? If yes, did you have one running when the birds turned away?