Not feeling the 'burn' during ab workouts

I'm currently on my second week of FB Abs Round 2. I'm not a massive fan of core exercises to be honest but I really need to strengthen my core. I'm hoping it will help me with my posture and to lift heavier, especially to protect my back when doing things like deadlifts :)

Anyway, when I do core exercises, I don't get the 'burn' that Kelli and Daniel talk about (and like I do in upper and lower body workouts) BUT my whole body literally shakes.

Does it matter that I don't feel my muscles burning. I definitely feel like I'm challenging myself and surely the shaking means it's working?

Sorry if this is a silly question! ;D