I'm currently looking into some unusual results from slush's pool payouts. Most blocks on which I complete work (shares) receive the appropriate payouts however some display shares completed but no payout amount.
It's obvious that you put Difficulty setting on pool's web-site to 128. That means that your miners will not send ANYTHING unless they collect at least 128 shares. Only after that they will send them to the pool to request some reward.
To resolve your issue - decrease difficulty setting to 1.
P.S. Yes, I know, that actually difficulty works in some other way, but it's the most easy way to explain its meaning and why its better to put 1 on slow miners.