First make sure the clutch cable is free. Take one end loose and slide it up and down. It would be a good idea to lubricate it with dry graphite (don't use oil or anything that would attract dirt).
While the cable is loose, make sure the clutch lever pivot is free. Also lube it with something that will not attract dirt.
If neither of those work, the problem is probably with the clutch plates. Has the bike been sitting for an extended period?
2005 DR, Corbin seat, Staintune exhaust & header, VAPOR, K&N, Suzuki rack and skid plate, LED and DR-250 taillight, LED rear signals, homemade mirror extensions
2005 Yamaha WR-250F Tagged