I've done the same thing, and my previous slurry took a while to start as well. I gave mine about a week, with frequent grav checks, and when i determined it wasn't going anywhere i pitched a new packet of Roeselare and standard Sacc yeast. Worked just fine.
In general, though, from what i understand, the Roeselare blend is a slow starter (cake or fresh pitch).
~~ Malted barley wants to become beer. ~~