We had a similar issue and I discovered that the invitees who were not receiving the emails has foreign characters, quote marks, in the name field of their contact information (e.g., First Name field listed as: John "Johnny" instead of just John). This was due to us uploading the list. Not sure if this has anything to do with your issue--sharing just in case.
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Alveeta Bower
Sr Specialist Learning Delivery and Logistics CMP
Cherry Bekaert LLP
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