Does the nintex forms for office 365 auto generate fields from List columns and if it does why it doesn't do this in my case?
On the first time opening a list the form should have fields present in the list. What type of list are you using? Can you provide screen shots?
If you drag controls onto your form, fields are not created automatically.
If you click the Create Column button from the ribbon in the Nintex Forms designer then the column will be created in your list and you will be able to connect to it with a control or drag the connected control from the List columns section of the tool box inside the designer.
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