I had the same issue for a long time. I couldn't add any mass, particularly on my arms, because I was doing a lot of running. My solution was just to cut way back on cardio and focus on gaining; but since that doesn't sound like an option for you, one of the things that I've done that has helped is to start taking BCAA's (amino acids) throughout my workouts, and even sip them throughout the day on my days off. This is a good, natural way to protect your muscle tissue from catabolization. Also, ditto what everyone here is saying about getting more calories than you burn. Eating is key, which I know can be hard if you're doing a lot of cardio. Good luck!