Your audience is becoming mobile and sophisticated. They want new content, as well as new ways through which they can stick to the webinar. For this you need to revise your webinar techniques.
Include Q&A sessions and other discussions as a perk for your membership sites. You can also organize paid Q&A sessions for your audiences with you or other experts associated with you.
You can promote your webinar via social media. Post something related to your webinar continuously on social media, so your audience stays connected to your event.
It is really difficult to get an audience to be a part of your webinar. You always need to look for a topic from your niche that is trending to be able to capture the attention of a large audience. Do some research and look for customer problems. Be relevant and informative with your webinar topic to encourage more people to attend your webinar.