Some remarks

• Your lockfile's path will be relative to your current working directory. I'd suggest using an absolute path to avoid miscommunications.
• Your sleep is called without argument, which means it'll sleep forever.
• Are you sure that a parallel instance should exit? Without LOCK_NB it would wait till the lock is free.
• You are not using parens for your arguments for flock(), precedence issues with | would make me nervous here. I'd need to look it up...
  it should work in this case tho.

Regarding debugging

You shouldn't rely on ps for debugging, append with >> to (another) absolute_path/logfile when your script is

• started,
• exiting,
• working,
• stopped,

including process ID and timestamp.

Please come back if you can really reproduce a problem.

Maybe of help:

File lock demo