Probably, because you have stopped mining at the end of the round ... compare the number of shares for similar round (or two) duration - for example 21402 and 21403 together are exactly the same time as 21355, so if you have less shares in 21355 that is the reason for your 0 reward.
This is the most common (and probably the only) cause with Slush pool. Some people do not read or understand the way it works, but then complain about nonexistent pool problems