My first thought would be: When was it last cleaned? And I dont just mean a boresnake down the pipe. Gas systems can be real finicky when dirty and it almost sounds like your shotty is short-stroking a touch. Do the shells eject strong and really fly or do they just kinda roll out of the port?
Elevator timing could be another issue.
We really need more info to get a more accurate picture of the malfunction.
How many rounds through the gun?
What condition is it in? Clean, dirty or??
What chamber and what ammo are you using?
The more info the better able to answer.