Inherited column from parent missing

I've posted here as well, I'm not sure what the difference is between MS techcomm and MS Answers.

https://answers.microsoft.com/en-us/msoffice/forum/all/content-type-inherited-column-missing/3669c2e1-3b95-4509-bbb9-2a4ceba8df44

The issue is, I have content types defined in the content type gallery, they are published and I can use them in sites. There is a hierarchy to these types,  

Approx 20 types inherit from 'Engineering Document' . There is a number field on this type ('Engineering Document') which on two sites so far, is not being inherited by the children. In the screenshot below, the left hand site is a view of a content type at the site level. On the right side is the same content type from the CT gallery. On the right hand side there is a field 'WFM Number' , but this is missing from the left. 

I don't understand how or why this is happening, or even beginning to diagnose. Even if I didn't care about the 'why' - how would I go about fixing it?

Thanks for any help you can provide.