Amazon buy shipping issue

Hi there wondering if anyone can help. Due to the change in rules regarding VTR im now using shipping directly through amazon. my issue is some products/items arent showing certain options when i buy shipping in bulk example no 2nd class only 1st class/RM24. whereas others are showing 2nd class fine. do you know what this could be down too? I use all of the same shipping rules on all items so now sure how this can be. any help would be much appreciated