The most likely cause is that you're using a master page, which causes the IDs to be mangled.
Mastering Web Control ClientID Values in ASP.NET 4 - 4GuysFromRolla.com[
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If there will only ever be a single instance of the control on the page, set
ClientIDMode="Static"
on your control.
If the script is embedded within the page, reference the control's
ClientID
property.
Otherwise, you'll need to find another way to reference the control.