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The scrum team performs following tasks

1. Development -- Dev Team


2. Testing -- Test team


3. Merging -- Senior Team [BOTTLE NECK]


4. 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.

Those "phases" you describe sound very non-agile to me, but there are instances where a single person cannot complete a story and needs help from another team member even in super-agile environments, so lets just assume it is indeed necessary:

Take those points and discuss them in the next retrospective. There is no silver bullet, you solution will look different from mine. But what you need to do is make it visible. Have a column on your board or a tag in your system or whatever you use. Make sure it's visible that there is a bottleneck and everyone knows about it in the standup. And if senior people decide that they have more important things to do, then the story will not get done. You cannot do more than notify the people responsible. Don't forget to lower the velocity when you did not finish tickets so you don't get the same problem next sprint.

After a while, the senior people will get into problems with management on why their team is delivering so slowly and will see the need to optimize this, too.

In an ideal world, they would see it from the get go, but unless you know an ideal company you could switch too, you will be stuck with the real world :)

The bottlenecks are known so the first thing is to bring them to a Retrospective, as was already suggested. Try to find the root-cause of why they're seen as bottlenecks. Why do they have to hold back the entire development process?

Otherwise, I would say that there's apparently an over-commitment. If there are bottlenecks, it's because it's been proven that the team has less capacity than the demand of the work thrown on them. So, reduce the Work In Progress (WIP) so that things can get done. Once bottlenecks are improving, increase the WIP limits as you go along.

I feel there is an issue in sprint planning itself because as part of retrospective meeting, we know the progress of previous sprint and team commitment, so this has to be addressed for the upcoming sprints.

Your dev team is too big.

More accurately, it is providing too many developer-hours.

This is a great problem to have; there's always other projects and teams that need more resourcing.