Are you not seeing the excerpt in your raw feed, or are you having trouble getting it to show properly in your MailChimp email?
If it’s the raw feed, you may have code in your theme or another plugin which is affecting the excerpts. If it’s the MailChimp template, you will want to double check your RSS merge tags. The merge tag to use for excerpts is *|RSSITEM:CONTENT|* (*|RSSITEM:CONTENT_FULL|* is for the full article/post).
One option which can help with troubleshooting your feed is to enable the Alternate Feed option in the plugin, which will leave the original feed unmodified, and create a new feed modified by the plugin. If you enable the Alternate Feed, make sure to update your permalinks, and then you can compare the two feeds.
If you would like to share a link to your feed, I can try to help diagnose.
