Hi,
one of my teams struggles with unfinished stories during last 3 sprints. Most probably this is lack of cooperation issue because often they take on new items when all the other tasks are done or taken by others. They claim that it's impossible to split the work into smaller tasks that could be done by more people in parallel.
On the other hand, they talk quite a lot when they face technical problems, maybe not pair programming, but surely "pair analyzing". And there were successful sprints when almost 100% of the items were done (we don't have sprint goal).