在ios uitableview中间歇性地获取json feed

I am building a simple ios app that attempts to show sequence of images from flickr. The problem is it sometimes works and sometimes fails and I am not sure where is it going wrong. Application has a master detail view. Detailview is not currently used and the problem happens in master view itself. Am setting up a refresh button on masterview and trying to refetch the data when user attempts to refresh.I am building a simple ios app that attempts to