title: scrapyd only runs 2 jobs at the same time
body: <p>I was very happy with scrapy and scraped with max. 150 jobs at the same time.
  I am using rotating proxies with no problems. I changed max proc a few times, and scraped along with 10 jobs because the server i scrape is not that strong.
  After some time, scrapyd only runs 2 jobs at the same time.
  I didn't changed anything in the scrapyd config or something.
  This is my scrapyd config, stored in /etc/scrapyd/scrapyd.conf. Others conf files doesnt exists.</p>
  
  <pre><code>[scrapyd]
  items_dir   = /root/scrapy/results/
  max_proc    = 0
  max_proc_per_cpu = 10
  jobs_to_keep = 4000000
  </code></pre>
  
  <p>As said, i didnt changed it over time. I scraped with 10 jobs at the same time with this config.
  Scrapyd daemonstatus says </p>
  
  <pre><code>{"status": "ok", "running": 2, "finished": 100, "pending": 7, "node_name": "..."}
  </code></pre>
  
  <p>I dont understand, why scrapyd stops doing what i should.
  I did everything i can do to fix it.
   I runned scrapyd in other directorys, cleared the items and logs of the old jobs, but nothing changes. I double checked proxys and the spider code.</p>
  
  <p>What could scrapyd stop running more jobs at the same time ? 
  I'm really desperate :s</p>