Unable to send keystroke "down arrow key"

My apple script contains following lines:

....
....
....
tell scroll area 1
							click pop up button 3
							--Clicking down arrow key once helps in selecting the Tentative
status.
							keystroke down arrow key
							keystroke return
end tell
....
....
....

These lines were working fine on iMac with Leopard OS 10.5.4. When I
used the above script on a mini Mac with Leopard OS 10.5.3 the line
"keystroke down arrow key" stopped doing its intended action (i.e.
pressing down arrow key).

Expecting this problem to be related to OS versions, I updated the OS
of the mini Mac to Leopard OS 10.5.4. But this has not solved my
problem.

Please give me some pointer for pressing the down arrow key.

Thanks,
SKT.