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 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="0xAAA943C84244BF338428AA48DA19"
Name="Proposal"
Group="Custom Content Types">
<FieldRefs />
ContentType>