Bear with me; this leads somewhere.

Let's assume that your customers have some financial value to you. Even if you aim to help them reduce debt, that's still the case.

Prospects have a value below that of customers, since they won't all convert. And this particular set of prospects (which is converting at a minimal rate) has the least value.

But ... you expend a lot of resources trying to communicate with these people. If you can learn how to improve your process of reaching / converting the most difficult prospects, then you save wasted resources such as time; you'd gain more actual customers; and you'd improve your conversion rate long-term.

Even the hardest-to-reach prospects therefore have some value, which you can put a number on.

So my recommendation is to invest money to both entice them to respond and to learn why they weren't responding.

Offer a limited number of gift cards to anybody from this group who responds to a questionnaire or an interview. Make sure this is not a sales pitch. Emphasize that you're just trying to understand how to reach people better.

If they weren't responding before based on fear of dealing with their debt problem or because they were concerned about facing an aggressive sales pitch, then maybe a $5, $20, or $50 gift card just for speaking about why they weren't responding would tease them out of their silence.

It seems to me that someone with a debt problem might appreciate a no-pressure gift card. And you'd learn enough to improve the content or method of your followup.