Button Click Event not Firing when using asp Required Field Validator on Cascading Dropdownlist

Hi,
I have a registration form which contain cascading dropdownlist for State and City . I am binding data to these dropdownlist by javascript(Using JSON concept). When I am using asp RequiredfieldValidator to these cascading Dropdownlist ,then Final submit button(ASP BUTTON) click event is not firing to save data to database.
Note:  I have divided this registration form into 4 catogaries(i.e.look like 4 pages.Actually there are 4 divs,only one div is visible at a time). Each page have a Next Button(HTML Button,but last Submit button is ASP Button). By clicking this next button I am hiding current div and showing the next div through Javascript.
In Javascript I am checking a condition like that:  if( Page_ClientValidate == true ){  //Some codes here to hide and show those 4 divs... }