Internal Test Leads Hung Up in Progression Flow

I've checked and re-checked the membership and flow steps of our campaigns but still have (internal) people who are getting caught up in the channel progression steps and can't figure out why. They've progressed in the email campaign to the point of filling out our form, but never receive the "thank you" email with the content link. The system only registers them as having opened the email, or as having clicked a link.

Any suggestions or steps of action I can take to figure out what the problem is??