If trial users don't get a "this is mine" feeling about the paid version, there's nothing for them to lose when the trial ends. No loss, no reason to pay.
You create that feeling with anything they build inside your product: their data, their settings, their results, their customizations. The more of themselves they put in, the harder it is to walk away.
One thing most people get wrong: don't count a trial as "days since signup." Count it as days of active use. Someone who signed up and disappeared for 10 days hasn't had a trial at all. Start the clock on usage, not the calendar.
If you sell anything recurring, this is the gap between a 5% and a 20% trial conversion.
What does a user actually make or build inside your thing in the first session? If the answer is "nothing," that's your problem.