My guess is that there's an oring out of it's groove.
Assuming that my early Ghost is how currents ones are...
There are three orings in the frame of the gun, that "hold" the barrel. The front oring is a larger cross section than the two rear orings. The two in the rear are in grooves on either side of the transfer port. Shine a light in there (without the barrel) and verify that youve got three orings correctly seated in their respective grooves. If any of the three are unseated, it could create what you're describing.
The other situation would be that your barrel securing grub screw isnt loosened enough....
Also, the .177 barrel won't go in without pushing the cocking lever back if you've got the .30 probe installed...