cart not adding extra money when customer is buying more than one item

I recently installed v151 about for months ago. Moderators, if this is posted in the wrong section, please relocate it..Thanks.

So when a customer adds more than one of the same item, the shipping charges are staying the same and not increasing.

Also, I have had a customer buy multiple items and it only charged me one charge. I did noticed that I had only usps small flatrate checked in the module settings, but, when I did sime more testing and added medium flatrate. This resulted in now givng my customer an option to choose paying for small or medium flat rate.

Is there any way to make this work properly? Maybe set a maximum amount of items per class of shipping...say two items for small, 3-5 items for medium.....just thinking outside the box...

Is there something else I missed and did wrong?

Thanks in advance.