For those in the same boat...
If you are using the latest version of JQuery, you will find that the Cancel buttons will stop working. This is because of depreciated code in /administrator/components/com_mycomponent/js/jquery.validationEngine.js
It produces the JS error...
TypeError: form.find(...).not(...).die is not a function
The die() method was removed in jQuery 1.9. See
jquery.com/upgrade-guide/1.9/#die-removed
Using an older version of jQuery will get you by (version 1.8 )
Or, for an alternative fix, on line 97 and 98 change
form.find("["+options.validateAttribute+"*=validate]").not("[type=checkbox]").die(options.validationEventTrigger, methods._onFieldEvent);
form.find("["+options.validateAttribute+"*=validate][type=checkbox]").die("click", methods._onFieldEvent);
with
form.find("["+options.validateAttribute+"*=validate]").not("[type=checkbox]").off(options.validationEventTrigger, methods._onFieldEvent);
form.find("["+options.validateAttribute+"*=validate][type=checkbox]").off("click", methods._onFieldEvent);Also, line 101, change
form.die("submit", methods.onAjaxFormComplete);with
form.off("submit", methods.onAjaxFormComplete);Admittedly - I don't really understand this code as well as I should - so hopefully I haven't busted something else with this change! Reply back if you know better...
Same sort of issue as
www.j-cook.pro/forum/19-new-tickets/6758...ve-is-not-a-function
and
www.j-cook.pro/forum/18-closed-tickets/6...button-does-not-work 
