The problem is being looked at, but unfortunately it isn't an instant fix. We'll let you know when we know what the problem is. I've subscribed using several email addresses and I'm receiving them, so the problem seems to be limited to a few email providers. Please bear with us while we work this out, the problem may be complicated.
Both users having this problem are using the same email provider.
In the meantime, you can view the threads you subscribed to in your UserCP, so it will be quicker to get to them than searching the forums. Hopefully that will help in the meantime.
Thanks