And the unambiguous version, when you are ready for it: ask for something that costs them.
Encouragement is free, so it is abundant. The signals worth trusting all have a price attached:
Money. Even a small amount, even a pre-order, even a deposit. The gap between people who say they would pay and people who take out a card is enormous and it is the whole game.
Time. Will you spend twenty minutes setting it up with your real data. A yes here is worth a hundred compliments.
Their data or their work. Will you use it for the actual thing this week rather than a test.
An introduction. Who else has this problem, and would you introduce me. People do not put their reputation behind something they think is pointless.
Any of those getting a no, politely, tells you more than five yeses to would you use this.
On your two who said they would use it and have not: go back and ask what happened. Not to pressure them, genuinely to find out. The answer is usually something specific and useful, and it is a question people answer honestly because the moment has passed and there is nothing to be polite about.