Script works in Script Debugger, but not a folder action - can u help?

Question

Hi
I have written the following script to run as as folder action and it works fine when running it through in debug mode in Script Debugger, but as a folder action nothing happens.
I wonder if someone could have a look and spot what the problem is.
Thanks
Nick
on adding folder items to this_folder after receiving added_items
set convertPath to "Macintosh HD:opt:local:bin:ffmpeg"
tell application "Finder"
set inPath to this_folder as alias
set outPath to this_folder as alias
set filesToConvert to (name of every item of inPath whose name extension is "m4a")
end tell
repeat with i from 1 to count of filesToConvert
set fileName to removeExtension(item i of filesToConvert)
do shell script POSIX path of convertPath & " -i " & (quoted form of ((POSIX path of inPath) & item i of filesToConvert)) & " -f mp3 -ar 44100 -ab 192k -acodec libmp3lame " & (quoted form of (POSIX path of outPath)) & quoted form of (fileName & ".mp3") & " -mapmetadata " & (quoted form of ((POSIX path of inPath) & item i of filesToConvert)) & ":" & (quoted form of (POSIX path of outPath)) & quoted form of (fileName & ".mp3")
end repeat
end adding folder items to
to removeExtension(fileToParse)
#display dialog fileToParse
set prevTIDs to text item delimiters of AppleScript
set text item delimiters of AppleScript to "."
return first text item of fileToParse
set text item delimiters of AppleScript to prevTIDs
end removeExtension

Answer 1

mmh, well, it seems something is awry with the triggering. other folder action scripts seemed to work fine, but now are intermittent.
i created the following simple script:
on adding folder items to this_folder after receiving added_items
display dialog "1"
end adding folder items to
sometimes it works, sometimes not.
what could be going wrong?