Why can't I order anything with a size from amazon?

I have a Mac Book Pro, as does my husband.

Whether I use Firefox, Safari or Chrome, when I want to order something like pants, shoes or anything else that has a size, I can’t order it! I choose the size and the color, but it doesn’t let me choose to add it to my cart! I click on “Add to Cart,” and it comes back “Cart Empty.”

Just for grins, I asked my husband if I could try this on his computer. He brought up Firefox, I signed into my account and – voila! – it worked just fine.

My guess is that it’s some kind of mystery setting. We have the same versions of the browsers on our computers, so I don’t think it’s that.

Help!!! Anybody?