The easiest solution would be making a bigger collision box (by the way, the collision polygon in your red line sprite is wrong).
Or put a bigger invisible sprite for swipe detection.
Or you can make a more complex system - on touch start remember the coordinates and time, then on every tick as the touch continues, check if new touch coordinates are on the other side of the line. If this happened quite quickly, then you can assume that there was a swipe gesture across the line.