User Control's Contents Not Created Or Even Parsed

Question

I have an ASP.NET 4.0 application with some simple user controls. The user controls work fine on the design surface (VS.NET 2010), but when they're loaded during application execution, nothing shows up.

Futhermore, the various child controls of the user controls are null when referenced inside the code behind for the user control.

The code behind for the user control executes as expected, but any reference to child controls are null.

It's like ASP.NET is never parsing the ASCX to create the instances of the child controls.

Ideas?