The team, including the Product Owner, is having difficulty focusing on a single objective. Improving the technique of setting a common goal can very much boost productivity and morale.
Find out if Scrum is even suitable for this team. If you can not stabilize Velocity in a few months, then you should consider using other agile frameworks, like Kanban to drive your development effort.
Reveal possible recurring bad practices of reallocating people in and out of teams. Maybe one team member is occasionally borrowed to another team, or maybe production incidents are preventing team members from finishing or even starting the Sprint PBIs.
