Expectations of federal returns for foreigners that worked temporarily

I worked on a J visa last year in the states, filed for a paper check return when home. I received the Californian state check return on my doorstep about two months ago but i am still awaiting the federal return which is the bulk of the total value. Is there a reason foreign temporary workers are still awaiting this? Should i be concerned that nothing has arrived. My friends that filed for a direct deposit still have not received their returns back either so i am not inclined to believe the return might be "lost in the mail". I would appreciate any advice or opinions from the community. Warm regards.