Shipping only recalculates during an edit when Shipping Rate Recalculation is on - and it has limits.
Check these:
Shipping Rate Recalculation is enabled (Retain → Post Purchase → Settings).
The order isn't multi-origin - recalculation isn't supported for multi-origin orders.
The increase clears your minimum charge threshold - smaller increases are absorbed.
💡 New items ship at the original rate unless recalculation is on, which may not match your actual cost.