Unable to create a Report File

I have a number of observations in my Analysis Log. With the occasional exception, I've been unable to generate a Report File. The webpage reports that there is an unhandled exception and provides a trace. I won't post the full trace here but the beginning is as follows:

Column 'RA' does not belong to table Table1.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Oddly, I have been able to create a few Reports when I include one or 2 targets - but not consistently.

Any suggestions?

Pete