Reverting Chart

Hi all,
I'm having a problem I've never had before...
After adding new data, I've always been able to highlight the chart and
then move the "blue lines" around the data for the chart to add the new
month's info. (Sorry, I'm self taught and don't know all the technical
terms..) Now all of a sudden, on one of my charts, when I move the blue box
and hit "save", they automatically revert back to where they were. It's
driving me crazy.
Any thoughts? Thanks in advance for any help!