In CATIA V5 ,while offseting the A class surface to B surface , some areas its coming properly ,, how to get that unfilled surface

Suresh if you attach some screen shot in your question we will be help easier.

yes definitely it will.
that's why, we are using close volume method. if that issues would have not came. we can directly give thickness to 'styling surface' (A class sufc). If the surface wont take thickness easily. we need to create B surface by maintaining 'uniform' thickness every where using generative surface design(GSD) tools.

And that missing surfaces (in b surface) while offsetting canvbe mostly closed with the help of 'multisection' by giving 'Tangency continuity' to surrounding surfaces and using spline. we can also use 'sweep', 'blend' depending on the situation. Recommended to use multisection, because it gives good finish.

Finally you surface should be smooth without any sharp edges to be a good cad design. anyway in manufacturing point of view it(sharp edges) will not matters much. but it should be modifiable at any point of time, that can happen easily only when CAD is smooth without sharp edges.

Hope it may be useful:)