When You make blend shapes you mustn't change the geometry in any way other than moving what is there (verts, edges, faces) so you need to make sure that the model that you will be rendering has the necesarry points to make any facial expression that you need.

Blend shapes in 6 easy steps...

1: Make your model

2: Duplicate it

3: Select the duplicate then shift select the original

4: Create blendshape node (Deform > Create Blend Shape) under the animation menu set.

5: Alter the shape of the duplicate by moving the verts etc.

6: Operate the blendshape with the slider under Window > Animation Editors > Blend Shape.

Hope that helps,

Mat.