To troubleshoot this, make sure that all required fields are represented on the form and that there is data for them to submit. Also check the Update properties of all of the cards and make sure that they capture the data from the control contained inside. If this is occurring for some but not all users, make sure that you have published the app after making changes and that they are using the correct version.
If the datasource is a SharePoint list, the title column is often the culprit. Check in SharePoint to make sure it is not required or if you are using it, make sure that there is Default data being saved to it.