Text boxes in xy plot

I have a large spreadsheet with lots of numbers and lots of xy plots. I
find if I insert a text box tied to a cell into a chart (select
chart,type=,select a cell,hit enter), from then on I can only select the
text boxes for formating; I can no longer click on a curve and format its
color etc. If I save and close the workbook and open it again, everything
behaves normally. What is going on?? Resource problem?? Thanks all.