Using JQuery in ASP.NET's master page

Hi all
i have an asp.net page content that used a master page. i placed a JQuery code in this page content, but it not work. but this JQuery code correctly work in other pages that not used a mater page file.
the Jquery file reference is true. i placed it in both the master page and the page that used it, but it not execute.
why it not work with master page?!?!?!?!
please hep me
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