Next button not able to be clicked

So some of my users and myself included are occasionally having an issue that the next button on the ordering process can't be clicked. It's there and it appears but nothing happens when I attempt to click it. This only happens occasionally and I can confirm that it has occurred in both IE and Firefox. Any idea what might be going on?

Hi,

Are you sure that it's not because there is something not filled in the checkout and thus the system don't want to continue the checkout ?
Or maybe there is a transparent element of your page going over the button sometimes.
But hard to say more. We would need to be able to see the problem to tell you more.