The problem resolved when I provided 'place holder' values in the executable and arguments fields. I was leaving them blank, since I thought that they would populate with the expressions I built for them in the expressions tab. But, it turns out they have to be populated with hard coded values the first time they are run, even if it's just C:\foo. Then, at run time, they are replaced with whatever the expressions resolve to.

though many sites advise toggling the Evaluate as Expression to true. It didn't make any difference in my case, whether true or false.