Guilin Sun
Ansys Employee

It is hard to tell if your settings are correct, and the monitor is at the right location with right size. Please refer this post: 

Ansys Insight: Why my simulation result is different from published paper or experiment?

if you still have questions please paste some screenshots from your settings.

Make sure the monitor is at the right location and only record data around the rough surface. It seems the plane wave source and periodic BC are correct.