Hello,
I am also having the same problem.
I am using WP is 3.8 and Participants DB Plugin is 1.5.4.
I have just created a signup form. I tested my form by filling out the signup form from the website and via Add Participant (as administrator from the dashboard). In both methods, I got the message “The new record has been added.” But, when I check List Participants, there are 0 Participants in my list.
I do have some really long field names. Some of them were over 30 characters – they seem to have been automatically truncated to 30 characters. I can edit the titles, but that does not edit the field names. How can I edit the field names to make them shorter? I deleted the long database fields, made new shorter ones, and tested my form again, but that made no difference.