Shipping, Discount and Voucher buttons not showing on cart?

I'm working on a new 1.5.1.3 installation where I've copied a lot of files across from an existing 1.5.1.3 installation. For some reason, the buttons on the checkout/cart page for "Estimate Shipping", "Apply Discount Coupon" and "Use Gift Voucher" aren't displaying!

The code in the checkout/cart template is fine - the section of code where it generates these buttons is the 'foreach' loop for modules and it's not been edited.

I've tried switching between the fresh 1.5.1.3 controller for checkout/cart and the slightly (and I mean slightly) modified one on my existing 1.5.1.3 installation but neither works.

The records in the 'extension' database table for order totals are just the core ones (shipping, taxes, etc) plus a Checkout Survey module, but this module works on my existing installation and enabling/disabling it on this new installation makes no difference.

Does anyone have any idea where the source of the problem might be?!

For what it's worth, I put some code inside the "foreach ($modules as $module)" loop to dump some text on the screen and it dumped the text five times... so it's obviously finding five modules, it's just not doing anything with them...?!

On inspection of the HTML source code on the shopping cart page, it is in fact generating all the (seemingly) correct HTML code for the Shipping, Coupon and Voucher buttons, they're just not rendering on the page for some reason! Firebug isn't displaying any errors - anyone got any idea what's going on?