Issue with Javascript on Post Back

Hi All,
I am facing an issue in my Web form. I am certain that the solution is as simple as it can be, but still I am missing this.
Here is the case ;-
I have certails dropdowns and textboxes on my web form. Display of set of dropdowns and textboxes is conditional so I have a javascript written for the same.
On Page_Load event I register the script as ;- Page.ClientScript.RegisterStartupScript(this.GetType(), "script", fnDropdownvisibleenableScript, true);
The script executes fine and gives me desired result when page loads for first time.
But the same is not executed when a postback happens.

What may be the issue? Please help me.

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