AGOL Notebooks feature.get_value returns None

[MaryEllenPerko]

Hi.  I'm new to scripting and agol notebooks so I hope this is an easy answer.  

My script uses feature.get_value to retrieve attribute values.  Like this:

layerid = feature.get_value('LayerID')
layername = feature.get_value('LayerName')
lastiserror = feature.get_value('NewError')

For some fields, a value is returned every time.  For others, "None" is returned every time.  All attributes have a value.  And all field names used exist in the feature layer.  

I cannot identify a pattern why some work and some don't.  I'd appreciate any ideas on where to look.

Thanks.