hello, also i suggest to check if the theme is using tabcontent.js or other javascript files and if the theme is older than one year, you also may need to adjust js code from:
op_runSS(
to
Onepage.op_runSS(
or any other code that references the opc's onepage.js has to have the prefix Onepage.
the op_runSS triggers ajax on specific events.
to see if you theme gives any errors (especially when you are logged in) you may need to press F12 key in google chrome and see if you get any errors like "function not found" and similar at google chrome console:
best regards, stan