I am in agreement with HC. It sounds like you have hit your plateau, and it will take a little bit of shock work to get the desired results. Below is what I do to consistently change up the workouts to shock my muscles out of plateaus.
I typically plan my workouts in 3 month increments or 12 weeks. When at the gym I try and do a minimum of 10 exercises during this period and have two separate ways of grouping them.
The first way is: Monday-Chest/Triceps Tuesday-Back/Biceps(Lower back exercises involve some leg work) Thursdays-Lower Body Fridays-Shoulders/Abs. Of those 10 exercises I try and sub one new one in every week to keep working different muscle heads. This is the workout where I typically see gains in mass largely due to concentrations on muscle groups. At the end of one 12 week cycle I take a week off and evaluate how it went and what I need to do a better job of for the next 12 weeks.
I typically do this during the winter when a lot of cardio is done playing hockey on Sunday, Tuesday, and Thursdays. On Mondays and Fridays I will either run 3-5 miles, eliptical for 45+minutes, or the stationary bike.
Something that is key to this workout is that Wednesday and Saturdays are reserved for rest and recovery. This could be another area that you may want to evaluate during a plateau. Those two days are huge for the muscles to work and regenerate.
My summer workouts are again based on 12 weeks of work, but I will switch to a full body workout 4 times a week. I am in the process of writing up what that will look like for this summer. Typically that means 12 different exercises that cover a variety of upper and lower body work. This time of year I typically lose a few pounds pretty quick because the change up from the other workout. I also tend to see more definition over mass. There is also more bodyweight workouts with pushups, squats, squat jumps, etc.
Cardio looks a bit different as well. I will run a LOT more this time of year because hockey isn't available. Hiking will also happen a lot. As a rule though, I like to run 4 times a week.
In ten years of lifting I have gained close to 70lbs of muscle mass. I am content where I am and don't have any desire to look like a linebacker in the NFL. I want overall health, so have quit worrying about how heavy I lift. The plateaus don't come nearly as often, but hopefully this helps.