How many posts are you providing to Feedburner in your Original Feed - what is your max-results parameter ?
(remembering not to exceed 150 posts, nor 1 MiB in total feed size)
If a particular 3rd party streaming services is ill-equipped to store all old episodes, even when they're gone
from you feed, that's a limitation of that particular service.
Then again, those services that can store all episodes you've ever sent them, could store them
even if you deleted your podcast or your entire blog, so copyright issues come into play.