Reports within a project are only available within that project, not in other projects. When saving a report in a project you may get the option to save it As a cross-project template. (This option is only available for reports based on custom reports.) This saves the report with some additional data in it and makes it available in all projects.
When you open a cross-project report in other than its original project, its filters and fields may not be 100% right: different projects have different properties (jobs, locations, delegations, custom fields, etc.) Therefore you shouldn't use the template report (the one you saved this way) on its own; instead, in each project where you want to use a report like this, load the template report, verify its settings, then save another copy of it within the project itself.