Going to need way more information, but the most important for something as easy-going as Wisteria is going to be fertilization. Are you adding any fertilizers? If so, how much? How often? What do your Nitrates and Phosphates look like? It’s not likely to be the cause with this plant, but what kind of lighting do you have? How many hours per day? Water Wisteria might be one of the most hardy plants in the hobby, so something is definitely missing for it to be melting.