I may have solved it - after looking carefully at the Json being sent up when adding rows, I found that I was camel casing the data.
So I was sending up
{ "rows": [ { "timestampUtc": "2017-04-07T07:20:47.8260389Z", "intCounter": 726, "floatCounter": 952.93173610834947, "flag": false, "Label": "Label 14" } ] }
where I really should have been sending
{ "Rows": [ { "TimestampUtc": "2017-04-07T07:20:47.8260389Z", "IntCounter": 726, "FloatCounter": 952.93173610834947, "Flag": false, "Label": "Label 14" } ] }
Hopefully this is the root cause of my issues!