How do you know, you've reached the right moment to be more aggressive in order to close a deal with a client, that has been postponing it for weeks?

For example, you are running a trial phase for a new product. The client is offered a two months free trial, in order to convince himself and his management that your services are genuine. BUT, after the trial phase comes to an end, and the deal has to be closed, he keeps brining new issues on the table. He does not to cancel the trial, but avoids to sign the contract.