Why is stream from usb camera frozen?

edit

I ran my example in Raspberry Pi 3 (Raspbian OS). I got frames from 2-3 cams. When running first, my example was ok. Then, sometimes, one of cams could not get frame. I think stream from usb camera freeze. Because it's ok when unplugging and plugging in.

My code:

cam = cv2.VideoCapture(index)

// Set fps, width,height for cam

while True:

ret,frame = cam.read()

or

rt = cam.grab()

ret, frame = cam.retrieve(rt)

cam.release

I used multi-thread for getting image from 2-3 cams. Each thread for each cam.