I had similar issues... initially I was attempting to run the proxy off of a rpi as I have had in the past, but proxy was not happy no matter how many I had running - and it was resulting in lower hash rates.

After going through numerous troubleshooting steps, I moved the proxy (temporarily) to a macbook pro, just to test, and it had full hash rate... I have ultimately moved them to another machine entirely which has higher processing & memory power, my feeling was that it was probably memory related.

The processor load wasn't great but didn't seem too extreme on the rpi based on the 'uptime' commands I had ran, it was running at about .50-.60 on avg if I am remembering correctly, with just the proxy running. But it could have been processing power issues as well or a combination of both.

I didn't troubleshoot much further once I got them stable on another machine.