Maybe you shouldn't weigh at home? How often do you weigh? It can fluctuate morning to evening and day to day so stick to the weigh in with your counsellor if this sort of thing is going to annoy you.
There are so many reasons for it sometimes you have a few slow weeks and then it speeds up, my losses are up and down all the time but I've gotten over that now and only ever look at the numbers over 4 or 5 weeks, never just one week.
If you are remotely under the weather you can retain water, if you have had too much salt too, so if you add salt to your packs maybe cut down. As has been said already its also really important to make sure you are having your water, and too much tea or coffee may make you lose water but then your body thinks it needs to store it and will retain it after!
Don't get hung up on any one week or weigh in, it will only demoralise you. If you know (as you do) that you have stuck to the plan religeously, sit back and rest in the knowledge that your weight IS coming off and the numbers will add up soon enough.
Some of us (me and a few others) don't lose quite as much, my average weight loss is probably about 2.75 pounds per week, but I have come to terms with that. I am a short person and my body just needs less fuel to tick over than a taller person...... Its still double (or more) the weight loss I could promise myself on any other diet.
You are right there is nothing you can do but stick to the plan!
Mags
xxx