I've had my little one do that a couple of times, it is pretty ridiculous! Now she strikes, I guess because maybe she's bored and relishes the chance to "hunt", but it was pretty funny when she just moseyed on up to the mouse and opened up without a care in the world. Of course, when she did that I had been giving her some prey that, in hindsight, was on the small side for what she should have progressed to at that point. Maybe if they feel like the prey will not be a challenge to swallow, they don't bother wasting the energy on striking and constricting?