UpdatePanel Postback causes button to be unresponsive

I'm working on Asp.Net 4.0 - a simple E-Commerce website.

For adding of items into shopping cart, I used JQuery Ajax to post to server side Web Method. On success, i will do __doPostBack(Updatepanel.UniqueID, "")

That updatepanel will have a onload event whereby .update() will be invoked at the server side.

The problem that i faced is, 20% of the time, the Add To Cart button will be unresponsive. If it's clicked, nothing is being invoked. I will have shift my mouse and click on it again to fire off the ajax post.

ANy help is greatly appreciated!