This may be due to the Communication preference set to the customer:
Go to the customer/work address. Click on the contacts tab, then click edit. You will now be able to edit the communication preferences of this contact.
If you do not have the option of a communication preference it is because you need to add this to the customer information. you will not have the option to send a message via SMS if you do not have a mobile number recorded against this customer.
Or it may also be due to the way in which your email settings are configured:
You may be using SMTP within Commusoft, sometimes this can become unverified or remove due to settings within your email provider. If you have changed your password or email settings (sometimes this can be done by your provider without your knowledge) then this can reset the SMTP.
Please note, while Commusoft can guarantee that we've sent an email (check the notes and communications tab on the invoice to see the audit of the email being sent), we cannot guarantee the customer will receive it. This is due to several potential issues. The customer's mailbox is full, the customers set you to SPAM etc.