Reply edited post request, not working

Hi

I have a post request, and when i relay it i can see a response that contains json based on what was in the post request. I used the editor within mitm to change some of the post values and then did a reply, which reported as ok, however the results are the same, which tells me it replayed the original post request not the edited one.

Anyone know what i might be doing wrong.

Cheers