Several things could be causing that...including, but not limited to; the size of the fax, the fax file name (e.g., does it contain ampersands..&), and the number of pages in the fax attachment.
From the RingCentral KnowledgeBase article:
To prevent fax attachment issues, ensure that the following are met:
Ensure that the attached files are supported. The file name of attached files should not include ampersands (e.g. &) or other special characters. For more information, go to
Supported File Attachments for Faxes.
Ensure that the attachment
does not exceed 200 pages.
Ensure that the combined size of all attachments
do not exceed 20 MB.