1, examiner's workload is heavy: the examiner can't handle all patent applications in time under busy work conditions. If your application status is "in line", it is probably because the examiner has not processed the application.
2. The application materials need to be further improved: in the actual trial stage, the examiner will conduct a more in-depth review of the patent application, and will find that the application materials have problems and need to be further improved. If the application status is "in the queue", it is partly because the application materials need to be further improved to meet the conditions of patent authorization.