A decision was made, I am not sure by whom, to instruct the pilot to select flap and droop in small stages rather than complicate the system by having an automatic interlock if one surface failed to move when selected. I think the navigator was supposed to look over the pilots shoulder and monitor the indicators
Yes indeed! 15-10-10 selection was always called with "Moving together, stop together'' confirmation on the flap/slat/TE flap 'cheese' semi-circular position indicators - as was 30-20-20 and 45-25-25. If you failed to call out, the navigator would immediately challenge "Check cheeses!".