Do you have a caution arrow at the top of the page when you log in to Tableau Server? If so and you click on those arrows, are you seeing data out of date errors? Those should point to workbooks that are not getting updated data. I'm willing to bet the subscriptions that aren't sending can be found through those errors. Often they need an updated password or connection settings.
Did you ever resolve this issue? I set up subscriptions a few days ago and they have never run. The status shows "never" just like in your screenshot. I can confirm that the reports are refreshing normally (they refresh every hour), but for some reason the subscription never runs. Since I'm not an admin, I'm wondering how I can troubleshoot this issue?