These kind of things,

 

the more you pay attentions to, the more you get.

Were you smiling?  Why were you waiting for them to initiate a conversation?  A leader does not wait to be spoken to.

More often, it's a human-nature thing, not racial.

Clerks' attitude is a reflection of that of the management.  So, if you sense a bad vibe, you should first question the quality of the goods/services.  It's a tell.  People who are proud of their goods usually have a good attitude; they don't need to distract themselves.

Be interested in his values, not his skin color.