With the few details mentioned in your questions, I think this is a problem of text appearance of your dynamic text field which is in a direct relationship with embedding fonts in your Flash project. To avoid such behavior, you can take a look on these links :
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Adobe Help : Embed fonts for consistent text appearance.
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Google Help : Embed fonts for a dynamic text field.
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Adobe Help : Embed fonts in flash (video tutorial).
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Adobe Flash Developer Center : Embedding fonts (old article for CS4).
Hope that can help you.