Product Owner is not involved enough into Sprint execution

Product Owner is not involved enough into Sprint execution, i.e. no approve for "Done" product increment (if User Acceptance Testing process it requires) or User Acceptance Testing process has changed 

Discovered by combination of metrics:

Time in Status, when equivalent of "Product Owner Review" status increases

Velocity: Committed vs. Completed, when team does not meet commitments leaving sprint backlog in not completed state by the end of iteration due to the fact that Definition of Done which includes PO acceptance on sprint basis cannot be achieved 

Average Velocity, when decreases or keep the same without any signs of productivity growth due to the fact that tickets are travelling from sprint to sprint not being able to reach DoD

Backlog Health, when low indicating that there is no enough "ready for development" scope to fill out several future iterations 

Recommendations: 

1. Introduce or enhance stakeholders management incl. engagement

2. Introduce or enhance product acceptance and success criteria

3. Enhance Sprint execution