Generally when you replace the friction plates it is recommended to replace the springs as well. They lose some of their oomph over time and if you consider how often the springs had to work in comparison to the life of the clutch you'll see why $30 for new springs isn't a bad idea. Also, the steel plates need to be ran through a media blaster or replaced. They need a rough surface for the friction plates to "bed" into. After stacking a new set of clutch plates it is wise to measure how tall it is compared to the clutch basket. Not all plates stack the same. If the stack is low it will cause slip, if it is too high the clutch may not fully release and cause the bike to "crawl" when the clutch is all the way in.
If the springs are worn they may be letting it all slip.
If the steel plates were not cleaned and blasted properly, there is a good chance the new friction plates have glazed over. Instead of being spongy the outside face is rock hard like a ceramic tile. Most times a light sanding or media blasting can remove this, if you are careful you won't wear the plates too much.
It only takes a minute to check the plates on a dry clutch. I'd recommend you pull them and visually check to see if the surfaces are glazed and whether the steel plates are mirror smooth or if they have some sort of light texture. I'll need to check some manuals before I can safely say what the stacked height is, but I recall that without the pressure plate installed you place a straight edge across the basket and using the backside of a slide caliper, measure the gap.
