I wonder if its because you're typing items directly into the DV's source box?

You could try the other options (eg: named/defined range) in Debra's page:
http://www.contextures.com/xlDataVal01.html
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Max
Singapore
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xdemechanik
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"Davieslondon" wrote:
I am having difficulty in adding new enteries to my data validation list,
every time I try to enter a new entry on the list it starts to over write my
existing enteries. Does anyone know if there is a maximum number of
characters allowed per drop down!? Or does anyone know how to get around this
problem!?