If you declare your Site Columns and Content Types in XML to deploy them as a Site Collection feature, you must make sure to ALWAYS include the <FieldRefs> node. If you leave it out then your Content Type will not have any of the inherited fields of the parent Content Type !!!
<ContentType
ID="0x0101004704AAA943C84244BF338428AA48DA19"
Name="Proposal"
Group="Custom Content Types">
<FieldRefs />
</ContentType>