If you go from a choice-list to another field using the tab key, the natural thing to expect is for the cursor to appear in the next field. However, there have been reports of this not happening. Going from a choice-list to a field, the cursor simply disappears. One of the main reasons why this could be happening is due to floating palettes. If the cursor is mysteriously disappearing, and you are using floating palettes, try closing the floating palettes, and see what the result is. This should result in the cursor appearing in the field.
The reason this happens is because once you tab out of the choice-list, the front most process in no longer the form, but the palette, therefore if you close the palette window, the cursor appears in the next field.
So, there are 2 solutions, you will either, have to write code to programmatically close and open the window when going in and out of choice-list, or manually open and close palettes.