A spec that has subspecs can currently not be used with the Podfile podspec directive. Specifically, the resolver can’t find subspecs in the same spec.
See this mailing-list thread for more info.
The current workaround is to use the :local option for the pod directive.
I think the podspec directive can be used with subspecs as long as one subspec does not depend on another.
[TargetDefinition] Don't returns subspec deps with podspec directive.