I suspect it might have something to do with pool's luck (statistics page -- 100% is normal)... I mine for mining.bitcoin.cz. There are good days and bad days... See the duration column in statistics. Anyway check if you have missed some blocks, see your BTC reward column -- the number should be +/- 10% the same otherwise you didn't have internet connection to the pool or some other failure prevented you from mining.

If indeed you mined the whole day non-stop it's the luck. Some times it takes few minutes to break a block some times hours. In the end mining bitcoins is highly about probability and in the long run you'll get some average. I too have witnessed some bad and some good days It's all probability. See https://mining.bitcoin.cz/stats/graphs/ the CDF graph and info near the end of the page and if interested follow the wikipedia's CDF link about how it works.

You'll experience (in the long run) these bad times with all pools (i've had some with bitminter too... don't even talking about solo mining -- all pools exactly counteract this bitcoin's characteristic). If you want you may read something about pool hopping, however mining.bitcoin.cz have some novel approach to kinda punish pool hoppers (see http://bitcoin.stackexchange.com/questions/5072/what-is-pool-hopping ) I totally do not recommend it because it's unfair. I do not do it however YMMW.

Bye