If you feel like you've been practising hard enough to have gotten past the point when you should be beating someone, then it's probably not your fault that you can't beat them. Along with what poopshark said, your teammates may also have stagnated or stopped taking the game seriously and you just need a better/more dedicated team.
But what poopshark said is the most likely circumstance, you're both growing at the same rate. If you merc often (for a team of similar level to your current one) and find yourself doing better against other teams or having better results while playing to your normal efficiency, then it probably is the environment around you, rather than your individual skill and knowledge.