Portfolio only shows the projects you have access to. If a project is missing, you usually only have access to individual projects, or the project is in a different department than the one you have access to.
Check this
Access level: Access can be given at group, organization, department or project level. If you only have access to individual projects, you only see those.
Department: If you have access to one department, you do not see projects in other departments.
Filter: An active filter in the table can hide projects. Remove the filter and check again.
How to get access
Ask an administrator in the organization to give you access at the right level under Settings. Access at department level automatically gives you access to new projects in the department.