Illustrious doesn't do well on grabbing from below the arms. Works at 0.7 strength, 1 if you really need the position to manifest.

V2

Pos: gsfb, sex from behind, grabbing from behind, shoulder grab, 
Optional: arm grab, w arms

V1

Pos: gsfb, sex from behind, grabbing from behind, shoulder grab, 
Use at pos if arm disappears: ((grabbing arms)), w arms