If you've noticed recently that you're not receiving forum notifications by email (it seems to affect some people but not others), it is being looked at and will hopefully be fixed soon.
In the meantime, there is a workaround that might help:
1. While logged into developerWorks, click your display name in the upper right.
2. In the expanded menu, click Settings.
3. Check your mail preferences and make sure your email address is correct.
4. Under "How often do I want to be notified about", change the setting for Forums from Daily to Individual, and click Save.
5. You can either try leaving it at that setting, or change it back to Daily. Be sure to save any changes.