I'm having exactly the same issue trying to find an isofix seat for my AH20. Tried a Joie Baby spin 360 GTI at John Lewis yesterday and found issues with the isofix being far back and quite low on the seat. The limited adjustment on the Joie meant the seat was inclined on the seat and brushed against the seat back when the spin function was used. John Lewis couldn't find any other seats in stock that had enough adjustment.
Anyone know any specific isofix seats that do fit (saw Britax mentioned though not sure what model)