Heyo,
We changed around a couple things with the ballp.it server recently, and one of the things that happened as a result was that mail stopped sending. Took a while to notice cause the only time I get emails from ballpit anymore is when somebody sends me a PM.
Anyway, if someone recently sent you a PM or there was a new post in a thread you subscribed to updates on, you didn't get that email. But you will now.