Ask HN: Want to make Swift learning materials convert better, but how?

We have a few screencasts that we are selling, but the conversion rate is not where we want it yet. I'm curious as to what people feel would make them more likely to convert to be customers and how we can overall improve our conversion.

Here's an example of a screencast that I feel should convert better than it does https://swiftcast.tv/screencasts/making-ios-apps-sexy

Should we have a preview? Does the free written version kill the conversion?

Look forward to all the feedback!