postback only works on the second click

Hello,
1) i have a image control on a page which is supposed to send url parameters to a url on a different server once it is clicked..
2) when i click it, it fires an event which uses the form parameters to build a url then this assigned to the postbackurl property of the image control.
3) After this first click the page is not redirected but if you click it a SECOND time, it goes to the appropriate URL.
Any idea why this is happening and what i can do to ensure it gets redirected after the FIRST click?
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