Why are my two other frames completely black?

edit

The problem has been solved. Play around with low_gray values and leave constant high_gray 155. And the max always be 255. You can play around with low_gray but don't set too high than 200.

import cv2
import argparse
import datetime
import time
import numpy as np

cap = cv2.VideoCapture('test.mp4')

while(cap.isOpened()):
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)

    high_gray = np.array([150,255,255])
    low_gray = np.array([20,50,199])
    mask = cv2.inRange(gray, low_gray, high_gray)
    res = cv2.bitwise_and(frame, frame, mask=mask)

    cv2.imshow('frame', gray)
    cv2.imshow('mask', mask)
    cv2.imshow('res', res)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()