Builds stuck in pending state drone

Question

I'm using drone:0.8 with the following configuration but the only two first jobs are succeeded in building. The rest of jobs are stuck in the pending state. I'm deploying drone using Docker Swarm stack deploy. Here is my configuration file:

version: '3'
services:
  server:
    image: drone/drone:0.8
    ports:
    - 8000
    - 9000
    volumes:
    - /data/local/drone:/var/lib/drone/
    environment:
    - DRONE_OPEN=true
    - DRONE_HOST=http://example.com
    - DRONE_GITHUB=true
    - DRONE_ORGS=my_fake_company
    - DRONE_ADMIN=my_github
    - DRONE_GITHUB_CLIENT=my_secret
    - DRONE_GITHUB_SECRET=my_client
    - DRONE_SECRET=my_drone_secret
    - VIRTUAL_HOST=virtual_host.nginx.com
    - VIRTUAL_PORT=8000
    networks:
    - edge
  agent:
    image: drone/agent:0.8
    command: agent
    depends_on:
    - drone-server
    volumes:
    - /var/run/docker.sock:/var/run/docker.sock
    environment:
    - DRONE_SERVER=drone_server:9000
    - DRONE_SECRET=my_secret
    networks:
    - edge
networks:
  - edge
  external: true

It's running behind nginx-proxy by jwlinder.