I would recommend using the free tools on DMARCIAN.com to take a look at why it may be failing. You can double check your SPF records (which are usually the issue here) to make sure you are in line with the new RFC as a lot changed in the past year.
The second area to check is the header on the message. What DMARC is concerned about that SPF did not look for is that the header is aligned. There are several levels defined here, but this should get you started. The hardest part of DMARC, is getting your vendors compliant, and making sure your marketing email vendors are also compliant. Good luck.
