Help Bibliography. natbib sort & compress errors

[vsharma]

I use LYX 1.3.0 on CYGWIN/Windows XP
I am using Bool{komascript}
I have in my preamble
\usepackage[numbers,round,sort&compress]{natbib}
the bib style is vancouver
Problem
If i site aaaaaa(1)bbbbj(2) ccccc (3)
in my document  and when i go back and add citation (3) before (1) thus 
aaa(3)aaa(1)bbbbj(2) ccccc (3)
i get in the output aaa(3)aaa(1)bbbbj(2) ccccc (3) and not

aaa(1)aaa(2)bbbbj(3) ccccc (1) as the order has now changed
what am i doing wrong
please please can anyone help!!
vivek