The team should be able to move the stories through the scrum board all the way to done. This should be without having to rely on resources outside the team. Is that possible with all of the phases in your process?
If yes, then how come the issues gets stuck and not completed? You should investigate and try to solve the issue.
If no, investigate what phases ...
From my experience, setting interim goal of individual user stories target date spread across the sprint duration helped. That would of course require each story to be well sized and individually testable. Coupled with continuous integration and delivery, it helped me a lot to overcome this problem while leading technical delivery team.
Also, as you are planning sprints and choosing who will do what, look carefully for the dependencies that are always present in software. We use Microsoft Project® religiously to help everyone see how everything fits.
"Longer sprints" are often quite a bit more manageable.
Do stories generally get done by the end of the sprint? If so then leave things alone. Tracking the sprint burndown always feels to me like micro-management. The team are responsible for meeting the Definition of Done for stories by the end of each sprint. Track the product burn-up (or burn-down) on a sprint by sprint basis, not on a day by day basis.
You need to look at that is happening inside of your sprint. A few things that might provide insight include:
How quickly are items moving into your sprint?
If everything is moving into progress on day 1 and 2, this may indicate that your team is trying to divide and conquer. In this approach, everything starts on day 1 and ends on the last day by design (...
Seeing things like "Ready for UAT" and "UAT" in a workflow always raises a concern for me. UAT is almost always outside of the control of the development team, and often even outside the control of the development organization. These activities shouldn't be part of a team's workflow if they are beyond the control of the team.
Two options I see:
A) Split your stories into smaller stories.
B) Make your Sprints longer.
If you're unable to get your stories done in a single Sprint, then either your stories need to be shorter (note the 'S' in INVEST) or your Sprints longer. Or both.
As always, since you're doing Scrum, bring it up in the Retrospective. State the issue, hear opinions, ...