Just solved a problem that I thought I would share.
Model field that must be Yes (used Inclusion validation with a list that includes the value 1)
Used a radio group with 2 radio buttons, Yes and No. Input Values 1 and 2 respectively.
Error message was not showing because the "name" property is on the radio buttons which do not present validation messages.
Set the config value "CombineErrors", which defaults to false, to true.
This will allow the form to show the error messages from the model validation on the RadioGroup or Checkbox Group.
Hope that helps someone out!