Please, post the script fired by the trigger.
This typically happens when some script step in your script causes the focus to leave the current portal row before your script attempts to access the fields from that portal record. Commit record is one common culprit, but not the only one.
Sometimes you have to make the first few lines of your script set variable steps to copy your portal field values into variables before the other steps of your script cause a change in focus.
It was very strange...I ended up deleting the field from the portal and then re-added it and assigned the same script to the script trigger. Now it works. Even though I did not see why focuses left the current portal row but Phil this seemed to be what was happening when watching using debugger. FYI, it seemed focus left the portal row before the first line of the script was executed. Anyway it works fine for now. Thanks for your help