When I first met Sosa Sensei, he asked me to call him Bill. I just could not do it. Not even in private when it was just he and I at his house. In my eyes he had my respect and deserved to be called sensei.
And everyone except his wife called him Sensei anyway.
I have met people of similar rank that I didn't feel the inclination to call sensei.
I guess for me it's the impression and energy I get from the person. Regardless of ethnicity.
The good thing is there are very few high ranking aikidoka that I don't have respect for.