Add missing columns after fields added to datasource

If your calling an API for your data and you receive records for each single row, you will have to unfold them in Power BI. When the datasets changes, these fields are automatically propagated to the dataset but might not be available in Power BI. 

The reason for this is simple. Unfolded records are not amended automatically in Query Editor. So you will have to do this manually. 

Adding the missing fields

Open the Query editor and select the table that's need to be amended. 
In the Applied steps sections click on the wheel after the unfolding action.

A new screen will popup where you can select all the columns to unfold.

Add the missing columns and click on OK. You should be able to use the columns in your visuals now.