SharePoint designer workflow stops periodically.

Symtoms : I had build a simple workflow to send email to a User Group.When I incorporated groups into the workflow-generated emails, it stopped working for almost everyone except me.

Cause:
Send Email action has SharePoint group(s) and/or workflow lookups in the To / Cc line. That SharePoint group has membership visible only to the group member.

Workaround:

Make sure each SharePoint group has at least Read permissions for the site. Additionally, that in the Group Settings area of the Group Page, under Who can view the membership of the group, Everyone is selected.

1. On SharePoint site, click Site Actions -> Site Settings -> Modify All Site Settings.
2. Under Users and Permissions heading, click People and groups.
3. Select appropriate group(s), click Settings -> Group Settings.
4. Under Group Settings area, select Everyone for Who can view the membership of the group, click OK.