Subdomain Question

[cbathd]

Having a difficult time on our site and looking for some advice. Our site pages are indexed perfectly, however, we have a subdomain where we have all of our images and PDF's. We only have the main domain set-up in Search Console with our sitemap. We can't seem to get any of our images indexed by Google that are in the subdomain however all the PDF's are indexed. My thought is to add the subdomain to SC and create a new sitemap that is just for the subdomain. Assuming we are not blocking any folders or files with our robots.txt can anyone think of any other reasons why the images wouldn't get indexed.
Thanks

[ThomasHarvey]

can anyone think of any other reasons why the images wouldn't get indexed.

Only thing I can think of is perhaps if your subdomain is serving wrong status codes for your images, unlikely but possible.