One would be language barrier, for example if I use an abbreviation such as the AICPA and you do not know what it means then you will not know what I am talking about.

By the way AICPA stands for the American Institute of Certified Public Accountants.

I agree with the previous 'postmasters.'

I would only add; lack of confidence, body language (eye contact, posture, etc), knowledge of the subject/s, failure to project, attentiveness, etc. And one of the most important, failure to listen and comprehend.

Stringer