It sounds like you could have mildew or botrytis infecting your cuttings. This fungus among us is encouraged by dampness, high humidity and cool weather. Use a quick draining rooting medium such as perlite for rooting cuttings and make sure the greenhouse has a fan for better air circulation. This is not the time to apply a high nitrogen fertilizer as soft growth is more susceptible to infections. Keep everything in the greenhouse very clean including your tools and gloves. Some greenhouse gardeners like to use disposable plastic surgical gloves when working in a cool greenhouse so that they are less likely to spread mold and fungi spores. Commercial greenhouse growers use anti-fungal sprays to control the growth of this common fungi. Owning a greenhouse has a steep learning curve so keep notes and try to learn from your mistakes. Keep growing, Marianne Binetti