Google Spam Filter Flakey

Is anyone else having this problem? I use gmail and their spam filter is usually quite good. However, every once in a while, like now, I go through several days where it lets through a bunch of obvious cialis/viagra messages. Not all, I'd say maybe an 15% of them, but that's still about 25 in the last two days. It doesn't make much sense to me since the messages are obviously spam and look a lot like the other messages it catches (they're from "Cialias/Viagra" how difficult is that to catch?). I do go into gmail and flag them as spam so it will learn but it takes a few days and then happens again a few months later. Is this just me?