Secrets of EV?

I thought I'd figured the whimsical behavior of "forced" EV mode. I haven't. We need more than 4 bars. We need low speed. We need a warmed up engine. Inexplicably Eve refused to come on at low or zero speeds with a hot engine and a high battery charge. After parking for a little more than half an hour, it worked again. Could it be temperature sensitive?
Does anyone really know the true secrets EV mode? And I promise I'm not trying to use it to run over old ladies in the mall parking lot.

 

Thanks. Gotta sit down and read that manual some time.