MVC emails getting stuck in queue

As we have brought a number of our sites over to MVC (K12 SP) have noticed some odd behavior with emails. The vast majority of emails (95%) go through fine but the rest get stuck in the queue and do not go out unless they are manually resent. Even though the system should be attempting to resend them if they are unsuccessful on the first try it appears as if it only attempts to send once and never attempts to retry. I know the messages themselves are fine because when you go to the Email Queue and hit Resend All Failed they will go through fine.

Any idea what could be happening here? Thanks