[Python] django, gunicorn, grpc: "Failed to pick subchannel" if run django grpc client with...

I am a beginner in the Django framework. I wrote an application consist of 2 microservices. One as a Django Rest Server and the other as a Django GRPC Server. the rest request translated to GRPC requests in the first server and sent to the second for the process.

------>|-----------|------->|-----------|
|rest server| |grpc server|
| | | |
<------|-----------|<-------|-----------|


when I ran the first server with python manage.py runserver iport the application works properly. but when I ran the first server with gunicorn myapp.wsgi --workers=1 --bind=iport, a grpc request from the rest server randomly counter an error:

{"created":"@1593330482.005402565","description":"Failed to pick
subchannel","file":"src/core/ext/filters/client_channel/client_channel.cc","file_line":3981,"refer
enced_errors":[{"created":"@1593330482.005396918","description":"failed to connect to all
addresses","file":"src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc","file_l
ine":394,"grpc_status":14}]}


the interesting thing is the grpc request was received by grpc server and it sent back the response.

wsgi.py:

import os

from django.core.wsgi import get_wsgi_application

os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'myapp.settings')

application = get_wsgi_application()

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