Inconsistent results in WebPagetest API

I’m testing a URL with WebPagetest API each 2 hours. The request is always the same one. However, the responses that I get are inconsistent, i.e. sometimes only the first view successes, sometimes both the first and the repeat, sometimes none of them (I’m looking at the tags successfulFVRuns and successfulRVRuns). Can someone tell me what is the reason of this behavior?
Thanks a lot.