@Kim2173 I think you answered your own question:
"or the once a month date"
My neighborhood in Berlin is also a bit of a mecca for street-art tourists, so I can see that your kind of experience tends to do well with a certain market. But people who take tours like this generally don't have the resources to take long, leisurely holidays - maybe they'll be in town for a few days at most, so it's just a lucky coincidence if your extremely rare tour date happens to be when they're available for it. Also, having only one review makes it hard to inspire confidence.
Knowing that you need more reviews under your belt, I'd suggest adding a few extra dates when you have time in the near future, and perhaps seeding the tours with friends who can make it a more sociable experience (lots of solo travelers choose activities like this in hopes of meeting people). Once you have an established track record, you'll probably find it easier to fill dates at a lower frequency.