Don't lose hope yet. I have noticed with mine that it is sometimes hard to see movement when the chicks start to outgrow there shells. You still should be able to see something most of the time, but it wont be as dramatic as it was a few days ago. And unless you are absolutely positive that the egg has died, I would still put it in lock down. I have had a few surprise me when I thought they were dead and the next thing I knew they were hatching. Good luck, stay positive, and let us know what happens. That's good news about your air cells BTW... you have made progress.