MP9 Bolt getting stuck open

[Magoo0511]

Hi all,

Hoping someone can help with what might be causing my MP9 to lock the bolt half open when firing. The gun will then vent air and not allow me to remove the mag from the gun until I disconnect the hose from the mag. Which then allows the bolt to go forward and the mag can now be released from the gun.
I use a Primary airsoft HPA mag
Also, have a CQB bolt installed
The rest of the gun is stock