The scrum team performs following tasks
- Development -- Dev Team
- Testing -- Test team
- Merging -- Senior Team [BOTTLE NECK]
- Acceptance Testing -- PO/BA [BOTTLE NECK]
The Dev and Test team finishes the tasks as per the plan however availability of PO and Senior team is an issue. Senior team is busy with their planned activities so they delay the merging. Sometimes the stories are not closed because PO cannot verify the story or find out the issues pretty late.
So during this period, rather than waiting for senior/PO, developers start working on next stories and that results in large number of committed stories while the delivered stories are always less than committed but almost constant from last few sprints.
Due to these dependencies and bottleneck the few stories spill over to next sprint. I am sure this is not the unique scenario but your experience might help me.