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.