Email notifications not working

[airbur]

I've noticed over the last few days that I sporadically get the email notifications of new posts for the topics I'm subscribed to. I get one for one topic, but then another doesn't come through. I can tell by going back to the topic and seeing a bunch of new posts that I wasn't notified of. I've checked all the user-settings and haven't made any changes. Also, I regularly check spam etc. and nothing is getting filtered into there. Any ideas? Thanks.