Muscles have to be "shocked" into growth. When you do the same thing over and over again, your body gets use to it, and has no need to grow. Mix it up. Also, if you want strength, stick with the basic movements (bench, deadlift, squat). Getting into a routine (ie. 3 sets of 12) is like driving down a long straight road. You mull your body to sleep. Try lowering your reps and increasing your sets. Alter the order in which you do workouts. Also growth requires fuel. You must take in more quality calories than you burn.
Too many people fail to build the foundation and just do isolation movements. You must start at the core and work out from there.
Also try pyramiding. 1 set of 12, 1 set of 10, 1 set of 8, I set of 6, I set of 4 taken to failure. The next workout (after the proper rest). Add a nickel to each side and try to knock out the same rep range.
You did what?