I have a some older technology Getwork miners working through a Stratum Proxy hashing on Slush.  Running around 50 to 70 GH combined

I had one recent round, 21228, where I had no reward for any shares.  What is up?  The value formula indicates the reward should have been 0.0013 BTC.  "The reward earned by a given user is given by the following formula:
(25 BTC + block fees - 2% fee) * (shares found by user's workers) / (total shares in current round)"

Not much loss but it seems like something is wrong or I don't understand the BTC reward algorithm, or I don't undestand reward reporting.

Round  Duration   Total Shares     My Shares  BTC Reward    Block #    Block Value   Validation
21230   1:43:31   728080133           102720   0.00390706   276139   25.04945416    98 confirmations left
21229   9:39:26   4091124633         480476   0.00383220   276127   25.12559563    86 confirmations left
21228   4:35:30   1928606238         102602   0.00000000   276067   25.17414052    26 confirmations left
21227   0:46:16   323420163             47502   0.00321315   276036   25.06559435    confirmed
21226   0:14:56   104058374             13282   0.00321300   276031   25.01980000    confirmed
21225   1:07:44   472925199             64496   0.00285957   276029   25.20153852    confirmed
21224   0:04:17   29652709               4292    0.00351801   276018   25.03507003    confirmed
21223   1:52:10   783569002             58652   0.00273363   276016   25.05123438    confirmed

Round 21224 sure was a winner!