output from flowservice gets lost

I have a directory where all sorts of files can be put. My job is to filter the ones starting with a fixed prefix, followed by a date and a time (separated by ). I have made an event that does a find on the first file like prefix*.txt. I submit it to a flowservice to extract the date from the filename. The output from my flowservice is a string like dd_mm_yyyy which I named dateString.
I subsequently do another find, using prefix*
{dateString}*.txt as file name. Works fine, dateString is translated into the date.

Next I perform a series of jumps to make sure I have all the correct files (should be 6 different files). If all are present I would like to rename them (keeping the prefix but losing the date), so pref1_dd_mm_yyyy_hh_mm.txt would become pref1.txt and pref2_dd_mm_yy_(anything) would become pref2.txt.

In order to do this I again do a find on the files (prefix*_{dateString}*.txt). Unfortunately, dateString has no value (anymore) at this point. Somewhere it seems to be lost.

As a beginner I am somewhat lost too. Any suggestions?