When scrum is new, everybody seems to enjoy it but after repeated routine of sprints, planning, and daily reviews it becomes in interesting and old. There are some early signs or indicators which can prevent this kind of situation:
1. Repeated
2. Unresolved issues in retrospectives
3. Stories are not demonstrable;
4. Unfinished stories are split and carried over the next sprint
5. During sprint planning, tasks are assigned to team members
6. Team members are not 100% on the project or are withdrawn from the project.
Read the complete article here: https://www.scrumalliance.org/community/articles/2016/may/when-scrum-becomes-stale-the-team-under-performs