ParseJSON - how many rows does it check

I have a datasource that comes in as JSON from a survey. If there is no response to a specific question, the schema name doesn't come back at all, as opposed to returning with a blank response.

I am using the ParseJSON function as:

= Table.TransformColumns(#"Removed Other Columns",{{"columnname", Json.Document}})

but this doesn't always give me all of the required columns.

I assume this is because it is only checking a certain number of entries in columnname before deciding which columns to parse out, and none of those entries contain responses to specific questions.

Is there a solution that checks all entries (there are about 1800 rows of data) before producing the relevant columns?