To diagnose better, you can capture the standard output and standard error streams of the external program, in order to see what output was generated and why it might not be running as expected.
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If you set each of those to true, then you can later call process.StandardOutput.ReadToEnd() and process.StandardError.ReadToEnd() to get the output into string variables, which you can easily inspect under the debugger, or output to trace or your log file.