>The form had been working fine. I made some simple modifications (added a couple of comboboxes), and all of a sudden, tabbing between fields does not work.
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>I've tried taking out the newly added stuff, but no luck. I've also ensured that the field references are correct. Other forms work as expected. The fields are enabled and not readonly (I can edit).
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>Any Ideas?

Do you mean that focus stays on the field when you press Tab?