What does it mean when someone does not hug you back?

Answer 1

It really could mean many things, some insignificant some more significant:

• they just don't feel comfortable hugging (this is just a preference)
• they very much dislike hugging
• they don't like being touched and/or touching (this could indicate they have been touched inappropriately in the past by someone)
• your hug startled/surprised them (that was your fault, be gentler next time)
• etc.

You will likely never know the real reason unless you ask them yourself.