What are you talking about is problem that you are committing to to much in sprint, thus, your velocity is inaccurate. If I can give you a piece of advice: not finished items should be marked as 0 work done and moved to next sprint, and thus it will affect your team's velocity in future sprints making you commit to less items, which should result in that eventually you will have real velocity of the team and you will know how much work you can complete in each iteration. Of course opposite is also true - with time you can get knowledge or improve codebase in such way that your velocity will go up.