Older posts link on category pages wont work

Recently noticed that the older posts link, on the category pages, now wont work.. (older posts on the author pages work ok) Not sure why so be grateful if anyone could help me resolve please? [I have just updated to 2.5.1 hoping this would fix it but unfortunately not..] Example of problem can be seen here: http://www.boxroomblog.co.uk/travel/