Let me explain...
They sell "more."
But...
Most people don't want extra emails in their inbox or extra "content."
And they especially don't want to pay to get more similar stuff that requires their attention.
Instead of offering extra issues or bonus content, offer a newsletter that's a completely different take on a complementary/related subject that your free subscribers will want to access.
Some people subscribed, but it didn't ultimately take off.
If you're trying to figure out what a complementary "DIFFERENT" paid newsletter might look like for you, let me know in the comments and I'm happy to give you some suggestions.