Difference in behaviour of a two-option checkbox between UAT and PROD

Hi,

We have a two-option (checkbox) control and have set it to run some code onchange.

Xrm.Page.getAttribute("new_checkboxField").addOnChange(DoSomething);

 This works fine in DEV and UAT environments. We ensured UAT was at the same state as PROD by exporting the single Default Solution from PROD into UAT. However, when we apply our code to PROD we find that when the user checks the checkbox, he/she has to click somewhere else on the form (i.e. make the checkbox lose focus) to get the "DoSomething" code to run.

What could be causing this behaviour?

Thanks.

Maz