This is a strange behavior I observed recently. Let's say you have a List with Multiple Lines of Text column. In the column setting, ensure "Append to existing values" is set to No. Now create a SPD workflow on this List and try to access the column value (just log this value to the workflow history list). When you create an item and put some value to this column, you can see the workflow history displaying the correct value. Now the not-so-cool part: Go to column settings, and change "Append to existing values" to Yes. Add a new item to the List. Go to the workflow history. Alas! The value is blank! In the SPD workflow, whether you set return value as "String" or "Plain Text", it always return blank! Summary: If you are dependent on the value of Multiple Lines of Text column in SPD workflow, ensure you have not set "Append to existing values" to Yes.