I experience the same with my website. Very few people donate, despite constant claims that everyone uses it all the time and that it made their lives so much better.

Such is life. I do the same with so many little tools that make my life better. Why pay for something if I can get it for free? There are so many things asking for my money. I say no whenever I can.

There are not many solutions here. Either sell your services, sell your users, or sell to your users. If people aren't upgrading to the paid version, the free version might be enough. You might need to adjust what they get for free.