C# - Server-side RequiredFieldValidators Not Working?

I have a form representing a survey that is dynamically generated based on some database configuration. I have a custom server control for rendering the survey (SurveyRenderer) which contains custom server controls for rendering questions (QuestionRenderers). I dynamically add RequiredFieldValidators for questions if they are flagged as being required. I add these validators to the SurveyRenderer's control collection.

The gist of the code...

// In SurveyRenderer.CreateChildControls()...
foreach (QuestionRenderer questionRenderer in questionRenderers)
{
if (questionRenderer.Question.IsRequired) [code]...

The client-side validation works fine -- if someone has omitted a required question, the validators catch it and the form doesn't validate. However if I turn off JavaScript and submit an invalid form, the validators do not seem to work.On the server-side I am calling Page.Validate() and checking Page.IsValid in the submit button click event handler. Despite submitting a form where required questions have been left blank - something that would be caught client-side - on the server-side Page.IsValid remains True.

// In SurveyPage.aspx...
public void btnSubmit_Click(object sender, EventArgs e)
{
Page.Validate();[code]....

Should I be adding the validators to the Page's Control collection, rather than the SurveyRenderer? How come it works on the client-side but not server-side?

UPDATE: My QuestionRenderer is annotated with:

[ValidationProperty("IsValid")]

And the IsValid get method is like so:

// QuestionRenderer.IsValid
public bool IsValid
{
get [code]...
If I set a breakpoint and step through, I can see that QuestionRenderer.IsValid is being fired OK. It is returning false when it should do. If I go fine-grained and call in btn_submitClick:

// In SurveyPage.aspx...
public void btnSubmit_Click(object sender, EventArgs e)
{
foreach (IValidator validator in Page.Validators)[code]....

So validator.IsValid is true, even though the call to QuestionRenderer.IsValid returns false. So maybe I haven't wired something up correctly? Is using [ValidationProperty("IsValid")] not enough?