Failed to open Shader Stream...

From times to times, I have some fatal error which are triggered by the following code:

Stream *s = ShaderGenManager::openNewShaderStream( vertShaderName );

   AssertFatal( s != NULL, "Failed to open Shader Stream" );

I do not get why sometimes it works and why sometimes it failed.
If someone can give me some hint, I will try to figure how to circumvent this problem.

Thanks