I posted the following in another thread but I thought I
should post it here as well since the issue has been fixed:
It turns out the issue was because I had <xs:include
schemaLocation="DocumentCommon.xsd"/> in the xsd file being
called in XmlValidate. Once I copied and pasted the code in
DocumentCommon.xsd into the called xsd, replacing the
<xs:include schemaLocation>, it worked.
So when I wrote in the original posting, "Not sure if this
matters, but the xsd does reference another xsd in the same
directory" , it turns out it did matter.