Custom button not getting enabled

Hi All

I have a requirement where in I need to add a custom tab in the standard opportunity screen. I am trying to create a custom button in that screen that performs a BO Action on click. But once the On Click event is assigned, the button is always disabled despite the 'Enabled' property for the button being set to true.

Kindly suggest a solution to enable the custom button.

Thanks & Regards

Jagathshree.