Usually this is caused by a JS error occurring that is causing all of your other code to fail. See if you can view the error console (F12 in IE) to see if it gives you any hints as to where the error might be occurring.
Mike and Jimmy ,
Thanks for your response .
I am using java scripts alerts which is very simple .
I have tested code before using it in NINTEX and yes debugged it as well using firebug in Firefox.
It does not shows any error but didn't work as well.
I have worked on earlier version of nintex and it works fine .But facing issues with NINTEX 2013 version 2.2.3
Do you have any expressions currently sitting on any of your actions, possibly the built in validation that rests on some of the controls? I've seen these interfere and do exactly what you are describing here when one is misconfigured, thus causing an error behind the scenes. The only way to tell is really to go through your controls one at a time and seeing what type of "expressions" you have sitting within their configurations and then removing those expressions until you find the bad one.
Yes ! for some controls i have used OOB validations .
Instead of that user wants pop validation message for related controls.