Why isn't Python OpenCV HoughP Transform able to identify all the spaced lines? - opencv

Question

When we have spaced lines on 1px. HoughP transform of python opencv doesn't mark all the points.
I used:
cv2.HoughLinesP(img,1,np.pi/180,400)
Theoretically it should be working fine be it dashed or non dashed. In this case it doesn't mark all the lines if they are on the same height.
HoughP Transfrom Sample Output
The Green Lines indicate the white lines that were identified.
I changed the parameters to this:
cv2.HoughLinesP(img,1,np.pi/180,10,10,10)
And got this output, as you can see the detection is still missing some parts. Its unclear how, for a straight line, a shorter line is marked but not a longer line.
*** After the method suggested!
After method suggested by Robert
Input Image: Input Image
Here is the code:
import numpy as np
import cv2
import time
img=cv2.imread("in.PNG")
img2=np.abs(img)
img=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret,thresh1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
lines = cv2.HoughLinesP(img,rho = 1,theta = 1*np.pi/180,threshold =
10,minLineLength = 10,maxLineGap = 10)
N = lines.shape[0]
print lines
for i in range(N):
x1 = lines[i][0][0]
y1 = lines[i][0][1]
x2 = lines[i][0][2]
y2 = lines[i][0][3]
cv2.line(img2,(x1,y1),(x2,y2),(0,255,0),1)
#cv2.imshow("Window",thresh1)
cv2.imwrite("out.PNG",img2)