I have had this phenomenon happen before and tracked it down to an old story of mine which went viral on Facebook. There was no bump in earnings because no member minutes.

If you've looked at all your old stories, this isn't your issue. But I recommend the earnings extension. This lets you seen exactly what stories are getting read in a day and separates our internal and external views.

https://chrome.google.com/webstore/detail/medium-earnings-info/eaalkfagdfokodepkfpcckcfballnedm