Trendline on Secondary Axis

Hi
A strange problem. In a workbook of many sheets, some chart sheets allow me
to plot a trendline on the secondary axis, and some do not!. I have tried
adjusting scales etc but to no avail. The trendline plots fine if the same
axis is used but disappears when switched to the secondary axis although it
still shows in the legend.
Anyone else had this problem?Thanks
Keith

I have a pretty complicated excel file with a dozen or so sheets.
When I added a secondary y data set, I cannot add a trendline. It shows up in the legend, but not on the chart.
I'm using 2000