During the execution phase of a project, a project manager notices that the project team...

The project manager should have determined the appropriate type of incentive for key team members. Not all team members are motivated by financial rewards. Some may value recognition, opportunities for growth, or other non-monetary incentives. It’s important for the project manager to understand what motivates each team member and tailor the incentives accordingly1. This approach can help improve team motivation and performance. Additionally, providing support and assistance, such as training and mentoring, can also help improve performance2. References: 

https://bing.com/search?q=What+should+the+project+manager+have+done+to+avoid+underperformance+even+after+distributing+financial+rewards%3f

https://factorialhr.co.uk/blog/managing-underperformance-at-work/