Sig_response error

I am using Web SDK V2 for ASP.Net Webform applications. I was able to implement it successfully for one of the apps, but having issues doing the same for another one.
The user is able to validate fine responding to the Duo push notification on the MFA Request page. But the sig_response item is not available on the MFA Response page. In the application that is working fine, I am receiving the sig_response item as part of the collection of posted form fields. But for some reason the same thing is not happening in another application. There sig_response is absent from the form collection.

I am implementing the MFA Request (the iframe) as a separate ASPX page and the response page (VerifyResponse()) as a separate ASPX page to keep things separate and neat. I am using the same set of settings IKey, AKey, SKey, etc for both apps. I have also tried using a different set of IKey and Skey for the other application to no effect.

I will appreciate any leads in the right direction.

Thanks.