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Your image #2 was too smaller 6060. You had before was 256256. There is nothing I couldn't do. Code:

#!usr/bin/env python3

import cv2 as cv
import numpy as np

im = cv.imread('holecan.png')
gray=cv.cvtColor(im,cv.COLOR_BGR2GRAY)
img = cv.medianBlur(gray,5)
cimg = cv.cvtColor(img,cv.COLOR_GRAY2BGR)
circles = cv.HoughCircles(img,cv.HOUGH_GRADIENT,
                          1,300,
                          param1=40,
                          param2=20,
                          minRadius=15,
                          maxRadius=30)
circles = np.uint16(np.around(circles))
for i in circles[0,:]:
    cv.circle(cimg,(i[0],i[1]),2,(0,0,255),3)

im = cv.resize(cimg, (320, 240))
cv.imwrite('hole.jpg', cimg)
cv.imshow('detected circles',im)
cv.waitKey(0)
cv.destroyAllWindows()

Output:

Your image #2 was too smaller 6060. 60x60. You had before was 256256. 256x256. There is nothing I couldn't do. Code:

#!usr/bin/env python3

import cv2 as cv
import numpy as np

im = cv.imread('holecan.png')
gray=cv.cvtColor(im,cv.COLOR_BGR2GRAY)
img = cv.medianBlur(gray,5)
cimg = cv.cvtColor(img,cv.COLOR_GRAY2BGR)
circles = cv.HoughCircles(img,cv.HOUGH_GRADIENT,
                          1,300,
                          param1=40,
                          param2=20,
                          minRadius=15,
                          maxRadius=30)
circles = np.uint16(np.around(circles))
for i in circles[0,:]:
    cv.circle(cimg,(i[0],i[1]),2,(0,0,255),3)

im = cv.resize(cimg, (320, 240))
cv.imwrite('hole.jpg', cimg)
cv.imshow('detected circles',im)
cv.waitKey(0)
cv.destroyAllWindows()

Output: