Spring airguns are hold sensitive. Some are very hold sensitive! You need to hold the rifle exactly the same way, shot after shot. Hands in the same place, same amount of pressure etc. A lot of people have this issue, especially with more powerful spring airguns, and often it is due to holding the gun like a heavy recoiling powder burner. A lot of people grip the stock too hard. Look up the "artillery hold". Also, when shooting from a bench, you ideally have to support the gun with your hand, not rest it on a surface. This should help you out - even with my old 600fps .177 Slavia, it makes a difference.
On the other hand, it's possible you had a dud! I've heard even the benji multi-pumps aren't close to how good they were back in the day.
B