Not working properly with Rails 4 / Heroku?

I've run rake asset:precompile but unfortunately I have a problem getting AJAX working on Heroku (works fine locally). The only difference in the logs I can see is this:

local:
Started GET "/users/test?page=1&ajax_section=comments"

Heroku:
Started GET "/users/test?page=1"

Any ideas why the "&ajax_section=comments" parameter isn't getting set in production? 
