syncing with zoho crm

syncing with zoho crm

Good morning we have moved over our invoicing to zoho invoice from zoho crm and have a linked system. Over the last 2 months we have found the syning to be poor and unreliable. As we speak I am trying email and invoice to a customer. I had created the customer in crm some months ago but only today put in their email.
I have then tried forcing a sync on the invoice end. written a new invoice in both the invoice app and crm but both say no email address?It is driving me and the staff made. For a great bit of software that helps us such a repeat invoicing and the ability for customers to pay online the sync issue is poor.