The pain point your problem is solving is not big enough. It’s a vitamin. If there was a problem with their painkiller or they didn’t have alternatives they would be hammering on your door. (note, I have no idea what your product or company does)

I'm afraid this could be the reason... But it's quite odd that people are willing to pay for the service I offer, but not to talk to me

Why do you expect them to? Did they have a bad experience for some reason?

I personally avoid talking to customer's service as much as I can. I'm more likely to give up on a product or switch to alternatives than say anything. There are a very few exceptions to this. I suspect many other people are like me.

I did expect them to talk because I read, and also listened in this video, that users are thrilled to talk with the founders of the product they use.

I can understand what you say about unhappy users, they are the hardest to establish a conversation with, in fact, I had yet no luck with them.

I also try to contact my most engaged and paying users, very little luck there.

So, either everyone is laying (very unlikely), or I'm doing something wrong.

Fair enough. That point they made sounded weird to me. I'd certainly not be thrilled to talk to the owner of any of the products I use. Although I'd like if the owner was involved when I'm trying to fix something.

I'm an engineer, so I can definitely think about products I use which I'd enjoy talking to the developers and designers. Sometimes they can be the owners too. But if they are more about getting a business running and started, I don't care about that stuff. So, I wouldn't personally want to talk. I suspect people who are interested in running a business would like that. So, maybe your users are not mainly from that segment of the population.

How much money (hourly) are you offering and how much does your product cost?

I'd be willing to accept $100/hr from someone I'm paying $100/month to, but I wouldn't care for $25/hr from someone I'm paying $25/year to.