offset on surface - weird , help ?

Hello all, 

so i'm trying to use the offset on surface but it's not working.

the surface is a lofted one from two edges (3d curves) and the distance between them is 55 all the way

the 1st pic is a successful offset, but only when i use 1.3 for the distance, and what get's offset is a tiny segment of the original curve !

the 2nd pic is the error i get when trying to offset - i.e. 10 for the distance.

help?

is the problem with the lofted srf - untrimmed ?

thanks

amir