It is OK if some of the items are not done until the end of the Sprint. What we have in the Sprint Backlog is our best guess on what we can deliver. If there's a pressure on the developers to finish everything, they will start adding margins
to their estimates which turns into less productivity because of the Parkinson's law. When the Development Team realizes they cannot finish everything, they get help from the Product Owner to adjust their work (tasks) to ensure they
will deliver maximum value by the end of the Sprint. Remember that they get help from the Product Owner to make better decisions; the Product Owner doesn't make the adjustments directly. The Product Backlog items in the Sprint Backlog
are not added or removed during the Sprint.