For any project, there are a large number of items that can cause problems to the schedule of the project, but not all of them are problematic at the same time. A risk or issue may be small or minimal at some point in the project and be much more significant at another stage of the project, and it is upto the project manager to have the current status of the project in hand at all stages of the project.
So, you have a situation where the project manager already has a listing of all the major items or issues that can cause a risk to the project and is also on the lookout for more such issues that could cause risk, and it is very important that the project manager highlights the risks that are significant at this point of time and brings them up to the attention of the management team of the product.
But it is not just the highlighting of the risks that is a primary job of the project manager. The project manager is not just somebody who reports the issues and risks of the project, but also takes the lead in trying to solve the risks / issues. For this purpose, the project manager needs to have a good understanding of the risks / issues and work with the relevant people for understanding how the risk needs to be mitigated. This needs to be followed by actually working out the mitigation plan (and if these are known risks or issues that were known even before they were actually a risk, then the mitigation plan would already be known) and ensuring that the mitigation plan is as per the actual risk, since even for a known issue, the actual mitigation plan depends on the exact scenario in which the problem occurred.
In some cases, the mitigation plan does not depend on only the team but may need help from outside the team. For example, there may be a situation where the team is behind in the schedule and needs more people helping out the team, and this cannot be solved by the team; the similar is the case when there is a dependency which needs a resolution from outside the team. In such cases, the project manager should bring this plan to the management team of the product where it can even be escalated to outside the team and to senior management if required. It is the responsibility of the project manager to drive this process and reach a point where the plan has helped in reducing the critical status items of the project to less than critical where they can then be managed to become a normal problem.