Focus server control with javascript

[Benny]

Hi,

I have a problem to use javascript to SET focus ON an ASP.Net server
control when smartNavigation is on.

This javascript:

document.getElementById("clientID").focus();

However, only it is not always working and I am not sure in what under
what circumstance it dose not work.

Can anybody tell me that why is it not working? And
Are there anyway to work around it?

Thank you very much