Just got a new digital TV .....?

and have found great reception on the usual four ogue channels ( cannot get Ch 5 in East Sus ). But when I try and auto tune the digital TV receiver in, it cannot find any digital channels although the Freeview site says my post-code can receive BBC1, BBC2, BBC3, BBC Childrens and BBC 24 hr NEWS. Can anyone please explain this to me? Thanks in anticipation !