Three of the problems with the garden that Sam notices in chapter 6, when he famously says that the "garden was turning back into Cleveland", meaning that the garden was taking on the problems of the real world and was no longer the idyllic paradise away from the world he hoped it could be.
One problem is segregation. Just like in the "real world", people are clustering based on race, language and ethnicity. While it's normal that people want to be next to people they already know, Sam feels it would be better if people got to know each other in this safe space.
Another problem is water. They don't have access to water so people have to bring water with them, which is difficult.
Stealing is a third problem. People take stuff from each other, so other people put up fences, which is completely against the spirit of the garden.
Th above answer is correct; the answer to your question can be found in chapter six of Seedfolks by Sam Fleischman. Seedfolks is a short novel told by a diverse cast of narrators, all connected to a garden in Cleveland, Ohio. The Gibb Street garden is almost a revolving door of characters, and chapter six belongs to seventy-eight-year-old Sam.
Sam is a former activist who wants above all else to help people and the societies in which they live, and he makes this garden his project not just to grow things but to change things. As the answer above suggests, Sam looks at the garden and sees the same problems there that he does in society. The garden for him is a microcosm of society.
One primary problem is a lack of water. There is no irrigation system in the garden, and people have to haul their own water in if they want to water their seeds and plants. Though we do not know what it is, Sam does hint at a plan to correct this problem.
A second problem is trash. The people in the neighborhood disrespect the garden by throwing their trash there. This is a problem that must be fixed in order to have a successful and productive garden.
A third problem is fences. People are trying to guard and protect their own parts of the garden from those who are stealing, and the result is a spirit of distrust and separation, two things Sam does not like to see here in this garden.
Perhaps the most significant problem that Sam identifies in this garden could be labeled segregation. He sees people working together but only with others who share the same ethnicity. He would like to find a way for people of all cultures, races, and ethnicities to work together for a common cause. He says:
I start up conversations in lines and on the bus and with cashiers. People see I'm friendly, no matter what they've heard about whites or Jews. If I'm lucky, I get 'em talking to each other. Sewing up the rips in the neighborhood.
He wants everyone to see beyond color or ethnicity, and he hopes he can make that happen in the context of the garden.
Though he has identified the problems, it is not clear whether Sam believes that they can be fixed.