GC portals often compress a real accounts-receivable problem into a short code or a one-line message. "Missing PO," "wrong project," "duplicate invoice," and "not approved" each point to a different investigation. Treating every rejection as the same email guarantees more back-and-forth and can move the invoice outside the current pay-app window. For a missing purchase order or project reference, compare the portal row with the signed contract, the schedule of values, and the most recent change order. For a duplicate invoice, identify the original submission and confirm whether the portal has merged, rejected, or parked the later file. For a short payment, compare the approved amount, retainage, back-charges, and change-order sequence rather than resending the same PDF. Save the portal receipt and the rejection text before making a correction. A corrected upload without the original record can make it harder to prove when the invoice was first submitted. If a waiver, lien right, or statutory deadline is involved, pause for legal review. Drumtap is a reconciliation and evidence workspace, not a law firm and not an autonomous sender. The dispute evidence guide explains how to pair each rejection with the smallest useful document set. The portal-recovery page shows the product workflow.