There is one common theme to all the answers I have read here and I completely agree with the...There are many reasons why we can stay on plan, do everything that we are supposed to do, things that we have done in the past and lost weight but for some reason unbeknown to us all, we stay the same weight or god forbid, even gain a few pounds. Our weight can fluctuate from hour to hour and I completely agree it is devastating to see the scale go up when we are totally expecting to see it fall.
There is however one thing I can guarantee if you get disheartened and give up, the scale will continue to go up and up and before you know it you will be back where you started from
Carry on doing exactly what you are doing and the weight will eventually come off.....Like Gagalu who posted above, I too have been doing this since 2010 and am so far down 136lbs....last week in particular I did everything I was supposed to do just as I have always done and expected a good loss, I saw a 1lb gain I have no explanation as to why I gained but I have just taken it on the chin and got on with it, that weigh in is now in the past and I can't change it I can only look forward to next week and continue to do what needs to be done.
Just hang in there you will get there and trust me it is well worth the wait and all the effort