Olark can take a bit longer to consider a chat session finished and to generate a transcript than one might assume in using Olark. Your Zap will trigger as soon as Olark pushes that transcript to the zap, but it might be slightly more delayed than you expect.
My transcripts aren't triggering the Zap
Make sure that your webhook is set up to send all transcripts automatically. You can do that by going here and making sure the "Send chat transcripts automatically" checkbox is checked.