It seems that after some time teams tend to find less issues to improve during sprint retros. First reason could be that assuming that the project context is unchanged and team is stable then there just isn't much that could be done after some point. Some things that can be fixed have already been fixed some e.g problems related to other organizations require continuous effort. Other reason could be that by maturing the team doesn't need initial framework of regular retros. It is able to address and solve problems as they occur and there is no need to wait for sprint retro. Of course there is also third option that team is just not able to assess changes that sneak in quietly but turn out to be big problems at some point (sort of like a frog that is put into water that slowly gets hotter and hotter).
There is similar tendency in daily meetings. For new teams these meetings usually take more time since people rely more on it for communication. As team members learn to work together better daily meetings will not bring so much new information anymore because everything that is important will be shared immediately not waiting for the next meeting time.