Bitminter - no rewards when more work was done?
When a block is found the 10 latest shifts get paid. Finding blocks is a random event. Sometimes we find the next block quickly, other times it takes longer.

Sometimes it happens that it takes a lot of work before the next block is found by the pool. With PPLNS what then happens is that some of the work is not paid at all. The opposite happens when many blocks are found in quick succession. The same work gets paid many times and the pay is extremely good. Over time this evens out.

This is the nature of mining. If you were mining solo instead of in a pool the variance would be much greater. You could find a block and get 25 bitcoins after a few seconds. Or it could take you several years.