This feature is available in all editions.
Several reports can help you pinpoint the problem:
The Sprint Dashboard gives you the data to determine which of these cases exists.
The Burndown chart shows the remaining work (to do) that was planned out before the start of the sprint.
The Detail Estimate Trend graph indicates any added task work that is identified during the sprint.
The Estimate Trend graph shows any increase in scope (newly added backlog items) in the sprint.
If situation A) is the case, you would see no increase in the Estimate Trend or Detail Estimate Trend, along with a less-than-required slope in the burndown.
If situation B) is the case, you will see an increase in the Estimate Trend, which would cause an increase in the Detail Estimate Trend and a slowdown or even a rise in the burndown.
There is also a third possible cause, we'll call situation C), where there is no increase in scope but the team discovers new tasks along the way that weren't identified during sprint/iteration planning. In this case, you'd see a flat Estimate Trend but a rising Detail Estimate trend and would want to help the team focus on doing a better job of identifying all tasks during sprint planning.
The Project Scope Change Report can point out which backlog items have been added, removed, or re-estimated if there are any changes in the Estimate Trend graph.