Can't select Field Calculator for some feature classes

[ConorMoloney]

To change the data type of a field, I've added a new field and I want to use the field calculator to copy the values in the original field. But for some feature classes I cannot select the field calculator. Does anyone know why this might be happening? [ATTACH=CONFIG]26487[/ATTACH]

[DanPatterson_Retired]

are you in edit mode?  if so get out of it and try again or perhaps the field can't be edited so check its properties

[ConorMoloney]

I found a way around the problem but you can't add a field if you are in edit mode. For some feature classes I could use the Field Calculator and for others I couldn't, which was unusual. I just added all of the fields first and then went into edit mode and used the field calculator.