InfoPath Form for Nintex Workflow 2010 does not work in subsites

I have created a Site Collection Reusable workflow using Nintex Workflow 2010 for SharePoint 2010 and have customised a InfoPath initiation form for the Workflow. When I run the workflow, it seems to work fine at the top web in a SharePoint Site Collection, however, when I try to run the workflow on a subwebsite or any deeper, I get the following error when trying to start the workflow:

A form template (.xsn) file cannot be accessed. You may not have the required permissions to open the file.