Scheduler: events are queued but not working

Hi

My scheduler was working till some time before. Scheduler is to send mail automatically…
I wrote it in hooks.py. “all” so it works every 4 minutes.
Now it shows
I understand that scheduler is queued but not working…

16:09:44 watch.1          | Wrote ./assets/js/erpnext-web.min.js - 4k
16:09:44 watch.1          | Wrote ./assets/css/desk.min.css - 264k
16:09:44 watch.1          | Wrote ./assets/css/module.min.css - 2k
16:09:44 web.1            |  * Debugger is active!
16:09:44 web.1            |  * Debugger pin code: 169-664-147
16:13:44 schedule.1       | 2016-10-26 16:13:44,407 - /home/amalendu/erpnext-AMC/frappe-bench/apps/frappe/frappe/utils/scheduler.py [DEBUG]:
16:13:44 schedule.1       | Site: amc.erpnext
16:13:44 schedule.1       | Queued events for site amc.erpnext
^[[A16:17:44 schedule.1       | 2016-10-26 16:17:44,753 - /home/amalendu/erpnext-AMC/frappe-bench/apps/frappe/frappe/utils/scheduler.py [DEBUG]:
16:17:44 schedule.1       | Site: amc.erpnext
16:17:44 schedule.1       | Queued events for site amc.erpnext

anyone know why this happened?