Daryl,
I don't know why, but sometimes the error messages do not pop up (Message Box or Validation Summary). I suspect it has something to do with AJAX, but I am not sure.
Please check the perfomance settings from your host settings (Host :: Host Settings): The Page State Persistence should be set to "Page" (and not to "Memory".
If that did not help (or if it was set to "Page"), check if you see any red stars (*) near one of the property fields. This field is then propably missing or has an incorrect value.
Best wishes
Michael