I had this problem a couple ot times with cramp balls. With the cramp ball outer shell it seemed as if the shell was stopping the ember spread.
The key is to think through how the ember will spread through your bundle. I find it needs to spread from your ember into a bit of dense, powdery, dry(ish) material before it will link to looser material. Have a look at http://www.bushcraftuk.com/community...ad.php?t=14445
Richard, London, UK
If at first you don't succeed - pause, reflect, change something and try again.