Hovering cursor over columns doesn't give values

Using Excel 2007 on WinXP. On several of my charts, selecting the chart and then hovering the cursor over columns does not display the values. This is especially true of columns on the far right of chart area of low values (toward bottom of chart area). See attached, "Devices" chart bars for G or Utilized bar for A. Same thing in the other two charts. Values for bars with higher values are displayed. Any suggestions?