A free sample is a tester, a test drive, a small taste of what I might expect to get if I enroll with you to experience your Promise of Value more fully.
That means your sample, whatever it is, must demonstrate at least one key benefit I will only get by working with you, rather than someone else.
For services, that kind of sample is not as easy to create as opening a pot of jam or giving away a spoonful of Thai green curry, but it pays to persist in looking for the right way to demonstrate the unique value you offer.
You may even charge for it. The point is to make it low-risk for both sides.
The important thing about a free sample is not that it’s free, but that it’s a true sample.