Podcast listeners to paying members — what actually converts?

I've been running a weekly podcast for about eight months and it's finally bringing in steady listeners, but the jump to paid membership is where I keep stalling. I don't want to throw a paywall on everything and lose the audience, but free-forever isn't paying anyone either. I know I'm generalising from a handful of signups, so I'd rather hear what's worked for others before I change the format.