Why am I not getting all my taxes refunded?

Please be advised that the refund is the taxes overpaid not the taxes paid. 

There are many factors as to why your refund is lower or why you have not had all your taxes paid refunded such as tax treaties, total income earned, time spent in the US etc. 

Be advised that even if you know someone, who got all of their taxes back, all cases are different and keeping in mind that as a nonresident you are not eligible for a lot of the benefits the residents can use, your refund is absolutely correct.

In case you have any doubts still, it is best to pass through all the steps again and make sure you have entered your details correctly.