Add to cart problems

I am sure this has a simple solution - I just don't know what it is!!

Dreamweaver CS3, E-cart 4.5.0, PHP - have no clue

I have a test page in the root directory of my site that I have been using to learn and setup e-cart. Everything about my testing so far has been working perfectly. Taxes, shipping, add to cart - it all works like a dream.

Yesterday I tried to expand this success to the rest of my site and started replacing my old PayPal buttons with e-Cart. My bubble was burst. When I click the button to "add to cart", it takes me to the shopping cart page as it should, but it doesn't add the item to cart. I have a page that just says "shopping cart is empty".

The only difference that I know of is that my test page is in the root directory, while my normal pages are in various sub-directories.

I went back to my test page and added another "add to cart" button, and it worked fine.

My redirect is correctly set to the shopping cart - it just doesn't add it to the cart. I looked at all the things I know to look at.