These are around the knee area. Without these muscles, your stance will be weak. The slow first set Tan sau , Fook sau sequence also develops muscles around the elbow area which you cannot see. Wing Chun power is hidden from view. We do not want large bicep muscles.
Repetition is the key.
Try to relax, slow down, be soft. Find out what is going on. Position, Sensitivity, Timing Speed and Power Position comes first by training the form accurately. Without proper position your hitting accuracy will be off and your power transfer will be reduced. Without proper position, you cannot neutralize force.