Workflow rule for anniversaries do not trigger

Workflow rule for anniversaries do not trigger

We want to send our clients and automatic birthday email each year and have the rule to trigger "Yearly" based on our clients "Date of Birth" so the way I understand it, is that this should trigger each year on the correct day and month since the information I read says it can be used to "ideal for tracking yearly recurring actions such as Anniversaries" but this rule does not seem to be triggering.

For example I set a rule up for a client who was born on 18th August 1973 so this rule should have sent a birthday email to the client in 18th August 2014 but it did not.

Does anyone have any idea why, or can someone at Zoho help here? thanks,