It sounds like you've created a calculation field using the Get ( AccountName ) function? If so it sounds like this field is set to store the result - try changing the storage options to "unstored" and it should work as expected.
If you're using FileMaker Pro v12 you can also do away with using a calculation field and use one of the layout symbols that you can insert via the Insert>Other Symbol ... menu command in layout mode. You can insert:
onto the layout in the header to achieve the same result.
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Found it. Thanks!