Look like your loading arm not lifting high enough. The top pic shows the arm is a bit too low that's why the shell stops in front of chamber. Take the trigger set out, check the carrying dog and the bolt contact area see if there is any damage or worn out. I'll look for some pics for your reference
http://airsoftcanada.com/showthread.php?t=162618&page=5
Check post #39 will give you some idea. Compare the carrying dog (pic 2) with loading arm, compare the bottom of bolts (pic 8) if anything different, any worn out or damaged? Compare the height of loading arm (pics 3,4,5) you will see your arm is low.