I found the problem:
menu SETTINGS>PREFERENCIES and there OPEN_GL
and there choose setting with SHADOWS YES.
http://sketchucation.com/forums/viewtopic.php?f=15&t=44020&p=392583&hilit=shadows+faces#p392583
As an alternative to 'flatten'... you might try to group all edges etc and then make a large flat surface below them, use the Sandbox tool and Stamp the group onto this flat surface ?