Seems a similar issue was reported on Powerpress too.
http://wordpress.org/support/topic/powerpress-having-trouble-with-authentication-for-premium-content?replies=2
What got my attention is
If you get it down to the basic common denominator, use the default user capability that PowerPress adds and make sure that user has that capability, then it should work. Chances are you are either using a different capability for the premium feed, or the other plugin has it's own capability (roles and capabilities) that you need to associate both the premium feed and the user to.
As I did not have the Premium Content role activated by Podpress, but had to add it manually as premium_content. I wonder if I spelled it out badly and it is now not working in the feed as the feed code is connected to another capability, maybe spelled premium_contents?
Let us wait for Ntm to give us his suggestions.