I agree it just needs some more passes or could it be since your used to the weight of the 6/8s you trying not to add any additional pressure as you shave and with a very light blade you may have to add a tad of pressure?

You didn't say what stone you are using but you might just try some more passes and see what happens. If you develop a wire edge then you know you've gone too far and its technique thats doing you in.