Normally, this is what should happen when a server validation error (due to domain model validation rules or microflow validation feedback) occurs:
If there are one or more widgets subscribed to validation messages for the attribute and the widget
If there are no subscribers -> show a dialog message with the validation message.
In case of a client validation error (type errors or form validations) a validation message is shown underneath the widget.
It there are validation errors, but no error message is shown either underneath the node or in a dialog, please file a ticket. However, having a validation error for an attribute which can not be edited by the user (because there is no widget showing the attribute, the widget is read-only, or the widget it is not visible) is a situation which should never occur.
If there are server validation errors, you could take a look at the request response with for example Firebug (in the console tab) or the Chrome Developer Tools (in the network tab) to find out what validation errors are preventing the microflow from being called. A red line with a 551 response code in the console indicates a validation error, and the response would be something like this:
{
"actionResult": "",
"datavalidation": [{
"guid": "2533274790395905",
"errorFields": [{
"name": "UserName",
"message": "This user name is already in use."
}]
}],
"instructions": []
}
Where "2533274790395905" is the id of the object, and "UserName" is the attribute.