Odd thought, but I wonder if the current robocall plague is (and will be) affecting polling during this election cycle? The result of relentless robocalls for me has been that I simply never answer calls from unknown numbers anymore. When unknown numbers were telemarketers and telephone survey people, I'd answer unknown callers because I could politely decline.
538 did some good analysis and explinations about this exact issue, but the articles that I can quickly google are from a while ago [0][1]. You can look over their analysis of different pollsters that they use and get a feeling for what 538 thinks is bias [2]. Despite some negative press in some circles, they tend to do a very good job of making sense of the maddness that is polling and they are very upfront about their own biases and issues.
