It's called luck, or variance.
You've just been lucky to find 2shares in a row within a few seconds in one round
and unlucky to find no shares at all within 30minutes in another round.
The rule of averages tells me that I get a share more or less every 10 seconds... I can understand that not happening during the 3 minute round, but in 27 minutes I should have generated some 203 shares on average.. the odds don't stack up. Surely something else must be going on? I see rounds like this daily.