I get questions all the time from people asking why they aren't making gains.
It usually goes something like this...
"I train with weights 4 days a week and also do 30 to 45 minutes of cardio 5 times a week. I'm not making size or strength gains. What am I doing wrong?"
In my humble opinion, cardio training and building muscle are diametrically opposed to one another. Doing cardio usually DRASTICALLY slows (or in many cases STOPS) muscular gains.
And ya know what? I don't think cardio is all that great for fat burning either. In many cases, you wind up burning up more muscle tissue than fat when you do a lot of cardio.
If you're doing cardio training and trying to gain muscle, try dropping ALL cardio for 4 weeks and see what happens. My bet is, you'll start gaining muscle mass and strength... as long as you're training and eating correctly to support muscular growth.