Coupon Code Field Not Appearing

Its been brought to my attention by a client that my coupon code field isn't appearing when the 'Have a Coupon Code?' link is clicked.

I've recently been upgrading the plugin without checking if its woking ok.
When I look at the html code using Firebug, it shows the coupon code div id but it is shaded out. i.e. not active.

I haven't made any theme changes recently, just upgraded the plugin a few times. I'm now using version 2.1.1

Any ideas? thanks for your help