HoughLinesP not detecting simple edges

Question

I've been struggling with a program to get an anki Vector robot to follow lines on the ground. I've got the problem down to the fact that HoughLinesP won't detect two simple lines (see edges.jpg generated by code). I've cut the program down to basics so as to offer it for comment. Any suggestions most welcome. And yes I've read the similar posts but they don't seem to help.

import cv2
import os
import numpy as np
import time

dev = 1
img = cv2.imread('temp.png')
grey = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
edges = cv2.Canny(grey,50,150,apertureSize = 3)

blank = np.zeros_like(grey)
maskArea = np.array([[(150, 250), (150,160), (450, 160), (450, 250)]], dtype=np.int32)
mask = cv2.fillPoly(blank, maskArea, 255)
maskedImage = cv2.bitwise_and(edges, mask)
cv2.imwrite('maskedImage.jpg',maskedImage)      #save masked edges for diag

while (True):
    lines = cv2.HoughLinesP(maskedImage,rho=6,
                theta=np.pi/180, 
                threshold=100,
                lines=np.array([]),
                minLineLength=10,
                maxLineGap=40) 
    print("============")
    if (lines is not None):
        radAngle=0
        for i in range(0, len(lines)):
            for x1,y1,x2,y2 in lines[i]:                        
                if (abs(y2-y1) > 10):         #select verticals
                    #print("====",x1,y1,x2,y2)
                    cv2.line(img,(x1,y1),(x2,y2),(0,0,255),2) #add line to image
                    radAngle += np.arctan2(x2 - x1, y2 - y1)
            #if (len(lines)>2): radAngle = radAngle/len(lines)         
            degAngle = int(np.rad2deg(radAngle))
            if (y1>y2): degAngle -=180
            print("degrees = ",degAngle) 
            cv2.imwrite('houghlines.jpg',img) #save image to disc
            disp=cv2.imread('houghlines.jpg')
            cv2.imshow('hough_lines', disp)   #display overlays on laptop
            cv2.waitKey(100)                  #refresh display   
    else: 
        print("None")
    time.sleep(1)