PPT 2007 series values on chart

[mooseman316]

I have a chart I created in PowerPoint (embedded chart or msoChart) that the series values are a named range in the datasheet (sheet1), but when I open it the next time the formula changes from
=Sheet1!Date (Date is the named range)
To
='Chart in Microsoft PowerPoint'!Date
Or
='Chart 2 in Microsoft PowerPoint'!Date
(When I have two charts on the same page or presentation)

These charts are not refreshing as the should when I just enter a new value in the datasheet (Sheet1)

Any ideas?
Does anyone know how to make the charts refresh?
Thanks