Why does iTunes 12.1 keep deleting podcasts I've downloaded but not listened to?

I have had this happen many times in the past few months. I spend quite a bit of time downloading all the episodes of a podcast so I can listen to them in my own time. Then when I refresh my podcast list to download my daily podcasts all of a sudden it will refresh and all my downloaded podcasts will disappear! It's very frustrating. Is there a way to get back the episodes I've "lost" and how can I stop it from happening again?