FIT field data recording sometimes doesn't record to FIT file

I have a datafield which records both a graph and data to the activity summary. It calls FitContributor.Field.setData() from within the onCompute() function (and the datafield initialize() function for data that is writtedn to the activity summary). 

It works most of the time but sometimes it doesn't record anything for the activity.  I don't have an explanation for this.  Twice it did not record anything for treadmill runs on a VA4S  F/W version 7.20, and a Fenix 7 F/W version 9.36.  Once for a run on the same Fenix 7.

Can anyone suggest a cause or something to investigate?