Use the same weights.

I workout with fitness blender for 4 years but I use the same weights:6kg and 2kg , I know I could lift more but I do not have bigger weight and I can not afford powerblock (I'm a student and I certainly can not buy them soon.) My problem is that i don't feel sore after workouts and weights are to light for me now.I want to build muscle but i have no progress.What i could do?