Flat rate and shipping classes

HI Folks and thank you in advance for any advice.

I have a flat rate set for all products without any shipping class. I have created a separate shipping class for smaller items to be shipped at a lower shipping rate and entered the amount under the flat rate shipping fields “Shipping Class Costs” but when I check out on a smaller item the cart doesn’t recognize the shipping class but instead charges the flat rate, if I add a flat rate item and a shipping class item it still doesn’t recognize the shipping class .So what am I doing wrong here.