Am I not getting in a good enough workout???

I've been doing the 30DS for about 4 weeks now. I know I should be on Level 3 by now but right now I can barely get through Level 2 without feeling like I'm going to kill over. I figured by now my endurance would have increased. I can get through level 1 no problems, i'm still winded but don't feel as winded as i do at the end of Level 2. I do know that the intensity does increase with each level but I'm just wanting someone elses opinion. About, oh, 6 months ago I was biking a few times a week, doing the 30DS and biggest loser cardio max and was able to do the 30DS and a session of the biggest loser cardio max. Might just sound like i'm rambling but just needing some opinions...Thanks everyone for your continued motivation and support :-)