What sort of contact print frame are you using? Holding the negative tight against the paper is key. The weight of a big piece of glass may be insufficient. I like my 8x10 prints better when made on my spring-closed contact frame. The one with the hinge on one end and thick glass gives fuzzy images. I use Rising Stonehenge and the find the surface wonderful. I haven't yet used Arches Platine for kallitypes but you might like that surface better. Also the color of the print is affected by the paper.
PS I learned the hard way to get a properly sized paper. Arches 88 feels like heaven but sucks coating solution like dry sand!