I think the first thing you should start with is to understand if there is less productivity or not, and then understanding why.
Can you see that the tickets closed are less during a similar time period last year? Does the same pattern follow all old developers, or is it the new ones that don't have the same productivity? Are the tickets actually equal as last year, or has complexity increased and therefore decreased speed?
If there is a general decrease in productivity across the team including the senior developers, and you don't see any specific reason, then I would suggest talkin to the senior developers, highlighting what you see and start asking their opinions on it, what do they see have changed.
So as bullets
1. Try finding out the reason why before taking action (and find out if there is an actual decrease). And trying find where the issue lie is it with individual or team, with skill or motivation, external changes or internal processes.
2. If you cant find the answers, start talking to the teams most senior developers
Can you see that the tickets closed are less during a similar time period last year? Does the same pattern follow all old developers, or is it the new ones that don't have the same productivity? Are the tickets actually equal as last year, or has complexity increased and therefore decreased speed? If there is a general decrease in productivity across the team including the senior developers, and you don't see any specific reason, then I would suggest talkin to the senior developers, highlighting what you see and start asking their opinions on it, what do they see have changed. So as bullets
1. Try finding out the reason why before taking action (and find out if there is an actual decrease). And trying find where the issue lie is it with individual or team, with skill or motivation, external changes or internal processes.
2. If you cant find the answers, start talking to the teams most senior developers