can be a little tricky to align & seat, especially with a barrel band. I've done mine with the barrel band removed.
One thing that I have found helpful for alignment is to slightly engage the breech / barrel set screw with the ports lined up. Make sure the barrel can still move but is not loose. Then align breech with tranfer port and then squeeze the breech and airtube tightly with one hand while tightening the hex head breech screw with the other hand. Then you can tighten set screw and finish installation.