When leveraging forms that use marketing automation with connections such as Salesforce, the forms may appear to be submitting on the front end, but no results are being submitted. If there is a lack of error messages being thrown this may be caused by custom fields within the forms.
An upgrade has been released within Episerver that allows for improved messaging in the forms. The site will require EPiServer.ConnectForMarketingAutomation version 5.5.0 and EPiServer.Marketing.Automation.Forms version 2.2.0 for this error display option to be present.
By default, "Hide Form submission error" will be enabled. When it is disabled, the form submission will fail and error will be displayed as shown here. Note that "Failed to submit" will be displayed by default, and any text saved using the error message field in the admin UI will be appended to it.