People or Group Field can not Search Users

Right now, I'm developing a company site with Sharepoint 2010

I have a people/group field on my list. When I try to search a person by their first name or last name, It gave me no result as follows

But when I typed username and clicked check button, people/group field accepted it and as result displayed his full name

Is it a bug on Sharepoint 2010? If there anyone who ever found this problem, please share me your experinence or solutions. Thank you