Dear Mr Patterson, 

I think the reason could be that you might have simply forgotten to add yourself in copy. In Settings > Account settings > Invoice sending, please check that you have indicated your email address in either the field "CC" or "BCC". This will ensure that by default you are in copy when sending a document. 
Or perhaps you can check if your spam box just in case it was actually sent to you but considered as a spam.

If however you think this is a recurring problem, please come back to me after granting tech access to your account.

Regards, 
Justine.