CRM Workflow rules

CRM Workflow rules

Hello,

I have set up a simple workflow to trigger an email based on a contact field {date} update/create.

I intend to update the field weekly in order for it to send emails.

I am testing it and It only sometimes work.
It will not send an email every time I update the field.

So I must ask (having searched the knowledge base),

Why is this not sending on each field update?

Thanks in advance
Alex