Visualforce takes over element Ids and often alters them. Have you checked the actual Id that is being rendered by Visualforce on your button? You may need to get creative with your Javascript to select your button.

Visualforce takes over element Ids and often alters them. Have you checked the actual Id that is being rendered by Visualforce on your button? You may need to get creative with your Javascript to select your button.

EDIT: One common way to deal with this Id issue is to use the jQuery EndsWith() selector:

j$('[id$="btn"]');