Aren't slicks and semi-slicks supposed to actually be better on pavement due to the increased surface contact? I would think that knobbies would be more likely to slip on pavement, especially when it's wet, because there's less rubber in contact with the surface. I've never had a problem with my semi-slicks so, like Wahine and Biciclista suggested, perhaps it is either the tire pressure or a matter of perception.