Both servers are set to 0 for max age. For example I was just downloading an linux ISO (oldest one I could find on one of the nzb sites) and primary provider was not finding any articles, but the client was not switching over to my secondary provider. I changed my secondary to my primary and now the articles are donwloading. (The nzb has articles from 2008).
I did change the setting (after the issue came up) to increase retries for articles not found but it was still not switching to the secondary server (provider).