Ajax on checkout page not working

The checkout process on my site no longer uses Ajax. Going from step 1 to step 2 causes a page reload. Once I am in step 2, I can’t click back to step 1. Clicking on this link does nothing:


If I remove the class “cm-ajax” from the a tag then the link works, but obviously causes a page refresh.

Ajax on the rest of the site appears OK.

Unforunately I can’t provide the site URL because it’s still in development, but can anyone help me with where I should look to solve the problem?