If the jury do not, in a special verdict, pronounce affirmatively or negatively on the facts necessary to enable the court to give judgment, or if they find the evidence of facts merely, and not the conclusions of fact, from the evidence, as established to their satisfaction, the court shall direct the jury to retire and return another special verdict. The court may explain to the jury the defect or insufficiency in the special verdict returned, and the form which the special verdict to be returned must take.