Praise be to Allah.
It is not sufficient in wudu to limit oneself to washing the forearm only, without washing the palm. When you have finished washing the face, you start by washing the two hands, then wash each arm from the fingertips to the elbows. If you washed them before you washed your face, the first washing is Sunnah, but washing them after washing the face is fard (obligatory). Whoever only washes the arms between the wrists and elbows has not completed the obligatory action that is required of him, so he has to repeat his wudu after he finishes it, or he has to wash the parts he has omitted, if he did wudu recently, so he has to wash his two hands and what comes after that.