More function causes problems with Categories

Hi
I have my site https://www.hifiideas.com set up with Twenty Twelve theme, using custom menu with a mix of pages and categories as the tabs.

I have used the more function on some posts, so far so good.

Problem is when accessing say via HiFi Review category tab you find a more function button as a reader, you click, whole post appears BUT the posts below that one are not HiFi Review ones, they are the ones recently written in comparison to the post you have just expanded.

Question

How do I fix it so that you see posts for the same category you started in after using the “more” function button?

Note I have more than one category.

Please assume you are dealing with an absolute beginner…

Cheers in anticipation
Dave