I feel a ranty, preachy thing coming on, so this could be a long post! I'll preface it by saying I went on my first diet aged 9 in the mid-70s, and I've been yo-yo dieting ever since, never getting below 11 stone in my adult life before putting it all back on again. So in one way it's deeply ironic that I'm about to launch into a lecture on weight loss, but in another way I do know what I'm talking about. 
I can't just read people beating themselves up about only losing a pound, or nothing, or gaining a pound and not say this... partly because so many of my failed attempts at weight loss have failed when I've had a couple of weeks of no loss or slight gain, thought, 'Right, that's it, there's no point in me depriving myself of all my lovely treats if I'm not going to lose weight anyway', taken refuge in food to make myself feel better, and hey presto there I am back at 15 stone.
So I've just done a calculation. Since I began this latest (most successful ever) weight loss plan just over a year ago, I've lost an average of 1.46 pounds a week, rounded to the nearest hundredth. But I don't think there are any weeks when I've lost exactly that!! A lot of weeks, particularly at the beginning, I could lose three or four pounds in a week. But they were always interspersed with weeks of losing much less, or nothing, or slight gains. Sometimes it was explicable - I'd gone off plan, or had made an advance plan to eat more than usual (e.g. Christmas/New year, and again on holiday back in July). But mostly it wasn't. Bodies are unpredictable, and often, for no apparent reason, I would go a week or two without losing anything, even when my calorie intake and activity levels were the same as the week before when I'd lost three pounds or whatever. In April/May I managed a whole five weeks without losing a single pound, and then with no change in eating plan or activity levels, began losing again. I have a bit of a theory that those of us who are/have been significantly overweight and have yo-yo dieted might have bodies that are even less predictable than the average, but I have no idea whether that's scientifically possible. However, what I have noticed is that lately my weight loss is doing what it 'should' much more predictably. I know that I'm taking in 3500 calories less than I'm using each week, and I'm now almost always losing the pound a week that the textbooks say I should (ha! I bet all that changes next week now I've said it...)
But I really want to say to those of you who are feeling bad about little or no weight loss or slight weight gain, please don't worry about it. Bodies do that. You're in it for the long haul, and there will be setbacks for no apparent reason. If you've stuck anywhere close to whatever plan you're following, give yourself a huge pat because you've made healthy but not always easy choices and you deserve to feel very proud of that, no matter what the scales say. There are three big things that have made a difference to me this time around, and one of them is accepting that my poor, addicted body which I have mistreated for so long is not going to meekly fall into line and do exactly as expected. So take a deep breath, hang on in there, keep making the healthiest choices that you can, and you'll see the weight go eventually. If it doesn't, that's probably the time to take medical advice.
OK, rant over!! Hope it's not too preachy.