If I have Polycythemia Vera without JAK2 gene, is there some cause that isn't diagnosed?

Possibly. The vast majority of PRV is JAK2 positive. There are other causes of erythrocytosis besides PRV. Evaluation by an experienced hematologist would be useful. Other causes of erythrocytes is include hemoglobin oaths, renal tumor, sleep apnea, lung disease, etc.