You could try taking out all but the body fields from the Tab order...
You could name the object (using the Position tab in Inspector) and gotoobject() in your script
The field in the subsummary does not have to exist... only a merge field. The real field should exist in the body.
In Behaviour are you able to go to the field in both Browse and Find modes?
My issue, is that the Set Field function is setting the field in the subsummary part
Your issue is not clear to me. Set Field  works at the data level: it sets the field regadless of where - or even if - it is on the layout.
I've tested doing the find manually on the subsummary field unsuccessfully as well so I'm assuming now finds just cannot be performed within a subsummary part.
I don't know of anything that would support such assumption.
I don’t think the problem is with the location of the field on the layout, i.e. what part. But rather are you trying to set a summary field and perform the find. “You can’t enter search criteria in …. summary fields.” From filemaker help.
A regular stored indexed field will be the best when trying to find records.
A very basic question Leah ... is your script putting the table in Find mode before performing the Set Field step?
I apologize for the delay! I got pulled into a different project and haven't had time to get back here.
Thanks for all of your input, I actually discovered that the issue was not with my script, but because I had not made the startdate, enddate fields I was using for the input fields for the range determination as "Global" fields. Now that I have made this adjustment it is working properly.