So this can be either :
1. There isn't enough pressure in the dump chamber to cycle the bolt. You fix this by turning the velocity adjuster on the back of the valve clockwise. Give it a quarter turn and pull trigger, repeat until it starts to work.
2. The sear isn't releasing. This isn't as likely if you took out the valve, bolt, and bolt spring from a working gun and put in this different, non-working valve. bolt, and bolt spring. But the fixes would be to adjust trigger geometry.