Here's the frustrating thing. Weight loss rarely happens in a smooth downwards curve - it is more often a series of stops and starts. Which means that you may not necessarily lose exactly the same amount every week - it could be 1lb one week then 3lbs the next. You need to look at this as an average of 1.5lbs a week over the time you will be losing weight.
Focus on keeping within your Checks allowance and your weight loss trend will be downwards, even if the odd week has a blip!