Have you created both the up and over buttons?
in other words, have you checked to ensure that you have actually added the rollover behaviour to your buttons?
otherwise in the fireworks preview, it may seem that you have perfect rollover buttons (you have also probably not checked the output from fireworks in a browser),
but when you export to html........it wont work.
wonder if that is the problem u r facing. If it isnt that elemental I may be excused; I am only trying to help.