Troubleshooting: how to see errors from folder action scripts
I've found it useful to put the folder action code into a template that can also be run as an applet, droplet, or from the Script Editor.  It can then be used for testing and/or manually run for chosen files:

on run -- applet or from the Script Editor
    doStuff for (choose file with multiple selections allowed)
end run

on open droppedItems -- droplet
    doStuff for droppedItems
end open

on adding folder items to theFolder after receiving newItems -- folder action
    doStuff for newItems
end adding folder items to

to doStuff for someItems -- do stuff with the file items
    try
        repeat with anItem in someItems
            -- whatever
        end repeat
    on error errmess number errnum
        display alert "Error " &amp; errnum message errmess
    end try
end doStuff