Worth checking the tail and nose diameter compared to the older pellets,see if there has been any change.
I still use an old batch (3yrs older) of RWS Superfields.The new batch do not shoot anywhere as good as the old ones,mainly due to the pellets(new) having change, being smaller on the tail dia and larger on the nose dia.
Possibly the air arms have a variation as well on that die 18.
Fighter against the 'Dark Arts'
Dark keeps moving in on the day.
Steadily going downhill faster!