I bought a Crosman model 1077 from Big 5 and tried using it with the Crosman co2 12g cartridge. When I shoot the pellet shoots 20'-25' and drops to the ground. It has no power. It seems like the pipe between co2 cartridge and the pellet is stuck. So I went back to the store and exchanged it to another one and the same thing. The sound of the air is week too. I can put my hand in front of the barrel and the air thats coming out is week. When I take out the co2 cartridge there is plenty of gas still. Anyone have any idea what it could be? Thanks.

Ah yes, mine did the same as well. Give it about 100 shots and it clears up. Don't ask me why.
It did it again when I took mine apart...about 75-100 shots later it was back up to full power again. Give it a try.