Shifting gears on my mountain bike - stiff shifter lever-Help!?

I have a Mongoose Quartz 18 speed Mountain Bike (model# R4773KM) The shifters go around the handel bars right next to the grips. The one on the left (for the the front derailleur) works great, very easy to turn, click, click, click, done. The one on the right side (rear derailleur) works fine going one way but very, very hard to move when trying to go up to the bigger sprockets. Down to the smaller gears sprockets easy, but real hard to move the gear shifter when going to the bigger ones. Any ideas?