What type of plan are you on? How much are you cheating on this plan?
Is this just a "walking on lunch break" diet or are you really working up a sweat, and doing it about 5 times a week?
I had the same issue a few years back. I took a pic of myself at the start of every month for 3 months. I would run 3-4 times a week, and workout on alternate days. Problem was is that I was still eating whatever, and drinking sodas and stuff. Turns out I lost weight but not a lot. It was like 5lbs in 3 months. Than after I saw the 3rd month pic I decided to cut my calories, and lost like a pound a day for about 15 or so days. got to my goal in 30 days after starting w/ lo-cal. Only had 20lbs to lose (back in 2002) but I lost it quick once I put my mind to it.