Extended Benefit no Benefit

My family and me are on classic comprehensive plan and have app**** for extended benefits for the therapies that are children are on. It was discovered that one of the practitioners did not complete the extended benefits form and so we had to pay out of pocket a large amount of money out of pocket to cover these expenses. So even though we have been approved now for these benefits Discovery is saying that sorry we will only cover from the date of submission. That is totally unreasonable. If Discovery believes the treatments are neccessary and have granted the cover moving forward why are they not prepared to show some leeway in paying for that which was clearly not our fault.