The first thing my drain cleaning system had, like this one:

https://www.skroutz.gr/s/14170730/Karcher-Kit-Apofraxis-gia-Plystiko-me-Mikos-15m-2-637-767-0.html

was without the hole, which resulted in it being unable to proceed when it encountered solid obstacles. I drilled a 1mm hole at the top of the ball, but while the problem was solved, it turned out that the hole was larger than it should have been, resulting in it slowing down the penetration. So I bought this ball whose hole is the correct diameter and I am waiting for the time to use it. I think that this hole was what was missing from the drain cleaner from the very beginning.