C# Can't reflect into private field

I got an issue.

In Unity I want to reflect into a private field. But I always get null for the fieldinfo. what am I doing wrong?

public abstract class _SerializableType
{
    [SerializeField] private string name;
}

// because I am using a CustomPropertyDrawer for all inherited from _SerializeType
public class SerializableType<T> : _SerializableType { }
public class SerializableType : _SerializableType { }

so using this method should actually work.

        // propertyPath in that case is "name"
        FieldInfo info = type.GetField(propertyPath, BindingFlags.Instance
                         | BindingFlags.Public | BindingFlags.NonPublic);

What am I doing wrong?

I am calling this method in a managed library that has its own CustomInspector. so it reflects into every field and figure how to display it. AppDomain is fullyTrusted. I don't know what else could be of importance...