Some pwm can't work on Hifive1

On Hifive1, PIN3-PIN6,PIN8-PIN11,PIN16-PIN19 are three groups pwm, I don’t know why PIN4,PIN8 and PIN16 can’t work as pwm? They are PWM1_0, PWM0_0 and PWM2_0. Also, on hifive1, these pins are not identification “~”.