There is no confirmed universal fix for Harpy's heavy attack failing to deal damage. First check the move information for your current Harpy Style and whether Davy is aligned with the target. Aerial attacks can miss when the enemy is slightly above or below the attack's effective path.
Do not assume that every Harpy action is meant to behave like a straightforward grounded heavy strike. A verified, move-by-move heavy-attack breakdown is not available here.
Identify the action you are using
Open the move information for the current Style. Keep its regular attacks separate from its active Style Skill: Harpy cores can grant different skills, so another player's tornado or projectile sequence may not be the move on your card.
Check the target at the same time. Being close on the screen does not establish that Davy and the enemy occupy the same height or that the attack reaches far enough.
Compare contact and damage
Use a visible enemy and watch the health bar or damage feedback after the action. Reposition to a clearer angle if the attack is missing. Compare that with a regular attack or the available Style Skill so you can describe which action actually fails.
This is a way to narrow the symptom, not a guarantee that changing position fixes every report. If the move consistently makes contact but still produces no damage, that is a different issue from an attack passing above, below or beside the target.
Keep useful details for a persistent issue
Record your platform, current game version when shown, the Harpy card's skill, the target enemy and whether another attack can damage it. Those details are more useful than labeling every aerial miss as the same bug.
Frank's Database can help review discovered Style information. No confirmed patch, required setting, specific button hold or save-reset procedure is established for this symptom, so this guide does not prescribe one.