Mike,
The shared variables will only work in a section below the one in which a subreport is executing, and therefore a chart using subreports MUST be in a section lower than the one containing the subreports. Since your subreports are in a detail section, that means the chart must be in a detail_b section (if the subreports are in _a), in a group footer section, or in a report footer section. Where you place the chart depends upon what you are charting.
If this still doesn't help, then please provide your group structure, how you are linking the subreports, and what you are charting on (the on change of value and the summary values). The general concept of what you are trying to do would help clarify the next step.
-LB