Like many things its a process of elimination. So I would:
- Borrow a friends tank to eliminate that as a possibility
- Inspect the hammer and sear for excessive/abnormal wear
- Inspect the trigger group to make sure everything is moving properly (springs attached, sear moves)
- Chrono the marker to see what the velocity is (too low = valve not staying open long enough = not enough blowback to reset the hammer)
- Go back to using your stock bolt if you aren't already
Rarely but sometimes excessive oil/grease can cause problems.
Your main spring should be roughly 12.5cm long, if your marker is left in a cocked position over time the spring can lose its springiness. If you are chronoing low, give your spring a little stretch, but don't over do it.