With help from the members here, I think I found the problem. I went out to check if the L brackets were contacting the A Frame properly and found the sides of my A Frame are very slightly concave and the brackets are contacting only at the top and bottom. You can barely see light through the brackets and the frame when they're bolted to the A Frame. Holding a straight edge on the side of the A Frame reveals about a .020" concave dip in the A Frame. So it looks like my options are to tack weld them on or bolt them through.