Transaction Update posts 0000 instead of variable

I have a transaction update node that updates AR20.1 user field with the work unit plus a text field established with a previous assignment variable that works most of the time.  Although the log reads the correct update was made but on occasion the user field displays 0000.  A restart is always successful in fixing the problem, but I need to resolve it.  I'm using 15 of the 20 available characters.  any suggestions?