I only get that problem if I am not careful to agitate the developer carefully as I add it. You are adding it from one location so if you take 20 seconds to empty the syringe and are not careful to spread it across the whole paper as you add it the that means one corner gets developed for, say, two minutes 20 seconds while the rest gets just two minutes. Make sure you are gently rolling the camera the whole time you are adding it as well as after.