I am trying to use the rolling hand method but sometimes the second note of the triplet doesn't always sound why
There are no triplet.
But you mean the middle finger:
Play the prelude slowly
Try to play it with bowed fingers and with stretched fingers. Find out the optimal stretching position of the middle finger.
Pay attention to the weight of the arms. Vary the weight.
As the middle finger is longer it is normally nearer to the torque of the keys and this would need more strength and power. Play in front of the keys. (You’ll see you have to turn the wrist.)
Play the arpeggio in a dotted rhythm.
Play it through by accenting each time a different note (=finger) of the basic figure (motif).
Play it f and as p and fast as possible.
Play it in different octaves. R.h. Very high and very low.