ICallbackEventHandler, RaiseCallbackEvent not firing when the page loads first time

[bs77]

I am using ICallbackEventHandler.RaiseCallbackEvent along with ajax modalpopup.The requirement is like when a user click on a image a modalpopup opens in which name should be entered.the user i/p is checked with db using callbackevent and the second modalpopup is opened to show the return value.But when the page loads for the first time and if the users goes thru the above procedure then the RaiseCallbackEvent doesnt work.After a complete postback by some other event,all RaiseCallbackEvent requests are executed properly..What could be the reason behind this ?thanx in advance