The yellow sticker on the box listed the fault as darts jamming, so I snapped it up to give it a new home.
I test fired the blaster when I got home
I grabbed my screwdriver and opened it up to find the cause of the fault.
Below is a picture of the faulty component:
This spring controls the plunger retention catch. In this case the spring was bent and jammed, so the catch stayed in the open position and the plunger was free to move with the plunger tube when the blaster primed.
I replaced it with a spring from a Recon clip lock.
Now the blaster fires without jamming.