What's happening...when I do my cardiac workout it takes me longer to meet my goal than previous weeks ago. 

For example, when I first started cardiac, I was burning 1400 calories for 1 and a half hour on the treadmill. I always had my incline between 10 - 15, for 80 minutes, and my speed ranged from 3.8 - 4.2 (depending on my mood).  And the remaining time I allotted for cool down and other such things.  But lately I have noticed it takes me almost 2 hours to do the same cardiac routine, without any changes. 

I do have a theory:  My sis and I was talking and she seems to think that since I weigh myself every week, I program a different weight in the treadmill, and it takes me longer to reach my goal of 1400 calories, because of the drop in weight...is she right? She thinks I should try a higher incline for a longer period of time or try to increase my speed for a longer time...is she right? 

Am I thinking about this correctly or do you have different theory, and can you suggest any changes so I can see the same results in the same amount of time.