Arduino motor shield

i have uploaded a sketch to my UNO mk 3 then mounted it on my 2 wheeled buggie attached the motor shield and all works fine but when I try to upload a modified version it will not load.I take the shield off and the program loads. I re-attach the motor shield and all works well. Why will a program not load when the shield is attached?
Any help will be much appreciated.
Regards
Philip Everard :slight_smile:

Shield interfering on D0, D1.