Full Item Issue with RSS feed to Mailchimp

I would like to have the full article sent within a mailchimp RSS campaign.

Looking at two websites I have, both with WP-Atahualpa 3.7.24. For one website (www.anncastro.net) the full article gets pulled into the mailchimp campaign exactly as wanted, for the other (www.dievogelschule.com) it does not. And I don't have a clue why. The code in the mailchimp campaign and the ATO RSS settings are identical.

Mailchimp's help pages state the <content:encoded> tags must be set. But WP-Help pages warn that that can be a problem. besides why should I need to manually paste those into articles when for the other WP site it is not neccessary?

I cannot figure out, what I am doing on the one site that is making the full article feed work and which I am not doing on the other site. Have any of you encountered this problem and can tell me where I should be looking?

Thanks,
Ann.