The reason that these techniques dont work is becaused the purpose behind them is often taught incorrectly. Most Wing Chun teaches that the movements are blocking techniques. Pak Sao is taught as a thrust, which is incorrect. The reason why is because it is against Wing Chun philosophy to exert energy by trying to out force your opponent. The moves are defensive in nature but not as blocking techniques. When used with a stepping movement the moves are used as slipping techniques.
Example: Your squared up in whatever stance your using, and working towards your opponent's outside, he strikes, you use Bong Sao to slip to the outside as you step forward and attack.
Pak Sao is great for slipping the jab if you step back. The object of Pak Sao and Bong Sao and other movements in Wing Chun are used for REDIRECTION not impact or force while speaking of defense.