Is it possible someone fills out the form without entering an email address or entering a bad email address (without an @)? That would cause the reply to to be blank and then it would default to replying to the from address instead.
Do you have a sample of an email that didn't work properly you can forward to me? I don't know what the cause is offhand, but if I had more clues to go on I might be able to figure it out.
Your settings look correct. The fact that it works sometimes implies it is something different about how the person fills out the form or maybe a different email client viewing the sent email that causes the problem. We would need to track that down to figure out the cause and then the solution.