I was having this problem for a long time, I try to solve it whit the suggestion the I found in some Post of the forum but no one solved my problem.

But finally I found what was the problem:

I had a MasterPage and his ChildPage, I had some ContentPlaceHolder and in the childpage I had in one of his Contents the ScriptManager, so I found that only in the content where I had the ScriptManager the WebDropDown was working fine but in other content it wasn`t working at all.

So what I had done was to pass the ScriptManager to the MasterPage.