Hi intersale09
That could be due to any JS error. Please, do open your browser's console and let us know if you have any error there.
I've seen similar things happening in the past and almost all of them were due to conflicts with 3rd party plugins, so I'd suggest you start disabling all your plugins except WooCommerce and ATUM and try in that way. If it works correctly, you can start enabling them back one by one until you find which is causing this.
Best Regards,
Salva.