Incremental PDT build "skipped due to error in required child"

[jo284]

I have had ongoing issues with incremental PDTs failing to build. I made changes to the sql trigger for my PDTs on 15th November, and after that, initially the tables built successfully (the last successful builds were at "inception"). The sql trigger is such that builds should be triggered once per day. However, since the original change, the tables have failed to build every time, displaying the error:

"Build skipped due to error in required child"

I notice that the trigger value is blank ('-') in Admin > PDT Details. However, when I look at Admin > Datagroups, the trigger values are present and as expected, so perhaps that isn't the problem.

The reference to a "required child" seems to imply some kind of dependency between tables, but my PDTs each only derive from one table in BigQuery (they are SQL-derived). There is a dependency between code in prod and preprod, but my PDTs are failing to build in both, each with the same error.

What might be causing this error and how might I resolve it?