I have a feeling the marker isn't properly balanced.
Here's some instructions to help you out.
With the marker de-gassed, unscrew the HPR and the LPR adjustment screws until they are zeroed out.
Stick a barrel swab or something woody or plastic-y down the breech that's thick enough to block the bolt from traveling too far forward. (Push the bolt back if you need to)
Gas up the marker.
Turn the HPR up to about 350psi.
The bolt should now be pushing against whatever you have blocking it. That item should prevent the marker from leaking down the barrel.
Turn up the pressure on your LPR until the bolt frees the item you have blocking it. If all is working well the bolt will stay back on it's own thanks to the pressure from the LPR that is keeping it back.
Once the item is free, increase the pressure on the LPR another quarter turn or so.
That should help get your marker in the ballpark.
If you can't get the bolt to cock back, no matter how high you turn up your LPR, then you'll have to tear down your LPR and check that out.