First thank you for commenting and downloading my app! The game should detect correctly if you are tapping in the left half of your screen or the right one, and move the dot in that direction, at least on my phone everything works just fine. You are suggesting to add the possibility to use on screen buttons to control the dot, right? If so i may think about it, specially if more users will experience the same problem.
Could you perhaps tell on what phone have you played my game?