I wonder if the tongue jack maybe under pressure of lifting the rear of the Tow Vehicle (TV)is causing some twisting. Here's something maybe interesting to check when you get back home. Try backing the TV up until the bars can reach the L brackets but before the tongue hits the ball. Now with the trailer not touching the hitch are the bars level? Try dropping the tongue jack of the trailer until you can just barely swing one bar on. Does the other bar go on easily as well? What if you loosen your added set bolt, can you get either bar to slide on first and difficulty with the second bar?