Assuming you are referring to a 3rd party scenery, and assuming this scenery comes with jetways ( and if it does, you disabled the GSX replacement jetway files for that airport after installing the scenery), if you are sure there are no scenery conflicts, then the issue is caused by errors in the scenery like orphaned links, unconnected paths, which has been already reported for several sceneries, which in some cases has been fixed by the developer.
