The problem: all of a sudden you cant edit all or some of your fields using sharepoint designer / columns have become greyed out in the settings.
eg:
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note some of the columns are greyed out, cant be modifed here or used in the workflow (you can read the fields but not set them)
The solution: re publish your form and go into every greyed out item, click modify, then click “allow users to edit data in this field by using datasheet or properties page” then republish
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What happens though when the “Allow users” select box is greyed out and cannot be selected?
At a guess, that is because some fields cannot be edited. for example people-picker fields since the datasheet view cannot display the appropriate form related view.