Activity permalink in groups not working

the permalink to 'view' an indivdual activity post does not work, instead it redirects to homepage. eg.
http://domainname.com/activity/p/2862/
This activity post was created within a group.
However, the view does work in a members activity eg.
http://domainname.com/members/member-name/activity/2846
Can anyone help?
danny