Please check the managed-schema file within your custom SOLR core.
This can be located for example here on SOLR VM:
This file should begin like this:
<?xml version="1.0" encoding="UTF-8"?>
<!-- Solr managed schema - automatically generated - DO NOT EDIT -->
<schema name="default-config" version="1.6">
If it does not start as above, then i suspect that you have not populated mananaged schema. This can be done by navigating to the control panel and clicking the link Populate Managed Schema.
Check your custom core config file. Is index all fields set to true?
< indexAllFields >true</ indexAllFields >
If not then you need to manually add the field.