United uses a different route to tap into NH inventory. They have programmed an interface to query the system NH uses because UA doesen't use the *A CITP. AC/Aeroplan just uses whatever is available to them via the common *A interface for *A partners. If NH doesn't make inventory available to that interface the online tools will not show inventory.

NH is completely moving to Altea by 2026, so there is hope: https://amadeus.com/en/insights/pres...nal-efficiency