We have changed the design of our download form (Form you fill out to download a white paper or a guide) inline with a site redesign.
It has been 1.5 months since we did this and the commercial guys say that the response rates have dropped significantly since the redesign.
The business have made a lot of assumptions as to what might have caused this reduction in downloads and want me to make everything better based on these assumptions. But i think we need to find out exactly what is causing the issue rather than guessing and making incremental/massive knee-jerk changes that could cause more problems than they solve.
So my question is; What usability testing method is the right one for this situation?