I have seen twice that my newly found 8-chains share not showing in "share value" display on the upper right of the ypool's page. Both times the display responded to 6-chains and 7-chains like clockwork. So it was not a problem with network or pool's down time. The XPM column in the personal stat page didn't have any change more than 0.001 after the above 8-chains were found. Not all 8-chains are missing. Often they do get counted. (Because my 8-chain/h is less than 1, most times it is the 6-chain/7-chains that contribute to the XPM column. So once an 8-chain is found you can see the result in the XPM column, if it gets counted.)
I am wondering what was happening. I am using one of the unofficial miners. Is it possible that the miner is secretly sending some of the more valuabe shares to another account?
More than likely what is happening (and you can verify this by keeping an eye on your miner's output) is that those shares end up invalid for some reason. Most often (for me) the cause of this is finding a share right as a block is solved, causing the share to become outdated.
I have checked if they become invalid by checking if there is a message and they were not invalid. The output info when every block is found also showed no invalid 8-chain shares.
What it looks like to me is that if you get a large chain, it doesn't register as a share, but makes subsequent shares worth more for a while
As I described I don't see any results of the 8chains in subsequent shares or XPM payouts.