Why can't I mix different shipping rates on my online store?

Hi,

On my store, I have 3 shipping rates.

A - Standard Delivery £5.00

B - 24 Hour Tracked Delivery £8.00

C - Free Delivery

Most products come under A & B, and C is only for a few products which can be posted as a letter.

If I mix a product listed under rate C with any other regular shipping product it will only give me the option for option A, and not the option to upgrade to option B.

If anyone knows what could be causing this I would really appreciate it. I have included examples below.

Thanks,

Rob

Example of shipping error.

Example of correct shipping