I've not seen it, but did read (once) that putting a felt wad on top of pellets seems to give quicker ignition. The fellow writing was concerned about sometimes detecting a discernable longer "burn" than at other times. Ramming a felt wad on top of the pellets "seemed" to aleviate the problem. This sounds like a similar situation, where the pellets are not burning rapidly enough, or are not carrying the flamefront to the next pellet as fast as should be.
This is all conjecture, so take it as worth the paper it is printed on.