Why do my subscriber counts fluctuate?
It's totally normal for subscriber numbers to fluctuate for both podcast and regular blog feeds. It's not uncommon for people to unsubscribe and then later subscribe again. Additionally, podcast apps have to be running in order to fetch your feed. For example, if someone subscribes to a podcast in iTunes and then closes the app, we won't be able to see the subscription until it's next refreshed.
Though RSS subscribers are a nice metric to have, per episode downloads are the most important metric to gauge audience engagement (especially for advertisers). You can see episode downloads by going to Statistics > Podcast downloads.
To ensure all of your subscribers are being counted, make sure you redirect the original feeds on your website to FeedPress. If you're using WordPress, search for FeedPress in the plugin marketplace, you'll find our official plugin. Install it and use it to redirect your podcast feeds to the corresponding ones setup in FeedPress (the plugin will make that very easy to do).