How do i get all my freeview channels to work?

i have a freeview box ive had it for over a year but i dont use it cos i cant get all my channels.at the moment i have no bbc channels and then i wil do another scan and get my bbc channels.i loose channel 3 and 4 and 5!!!how do i get all my channels.or do i need a special kind of ariel.thanks.