Email notification on my iteration plan are working for some, not for others, and sometimes not always.

This is what I've done to debug yet:
- I've ensured the team have valid emails
- I've ensured they're subscribed to the iteration plans
- In "My team area" / "Mail Configuration" I've ensured all checkboxes are enabled.

Moreover and in my case:
1. For iteration plan "X" I received all workitme modifications
2. For iteration plan "Y" I only receive notificaton to workitems I have created

I believe the issue is due to configuration and around the observation in #1 and #2.

Any help would greatly be apreciated.

Regard,
Yvon