SP Estimating Reviews & Product Details

In our current project we are 6 developers and do absolute time based estimation i.e. 2 SP is one day and Sprint duration is 4 weeks(20 business days). So we plan approx. 200 SPs (according to calculation 240 = 2*20*6) with 40 SP with buffer. But we are noticing from last two Sprints that we are able to finish approximately 160 SPs and now the Product owner wants to know the reason for decrease in the output for the last Sprints as it is clear that even with buffer we should be able to achieve minimum or approximately 200 SPs. How do i handle it?

Secondly I am planning to move to relative based estimation using Fibonacci series. But my question would be how to take a reference Story i.e my 1 or 2 SP user story for relative estimation? I believe here again the reference User story will have to be estimated using time i.e. say this user story we need 2 days and there on estimate other relatively.

Thirdly and lastly we have the following structure for ticket formulation:

User story i.e. Feature ticket

- Sub task 1

- Sub task 2

Currently with absolute time estimation we estimate sub tasks and sum up the SPs and update the major user story ticket with the sum. Is this a good option or are there any better way and also if i move to relative estimation how should i carry out the estimation for the above ticket structure. Review collected by and hosted on G2.com.

Manual errors and time consumptions. It helps the SP Estimate to work smoothly by the automation Review collected by and hosted on G2.com.

Only for trying to bill for project and justification Review collected by and hosted on G2.com.