multi product orders: weights not added up for shipping totals

All our products have shipping weights assigned, we don't have downloads.

Under Store Settings --> Shipping, we have selected "weight rate" and we have 9 layers of weight ranges: .01; 16; 31; 50; 70; 100; 150; 200. When we recieve an order of more than one item, each product given the shipping weight of one pound product, the shipping charge for the whole order remains at the lowest weight range's fee.

What are we doing wrong?