When people are struggling to lose/gain weight during a cut/bulk, there are several explanations for why that is. Unfortunately, one of the most common explanations is simply that they're wrong, they are indeed losing/gaining weight! They just don't know it.
This problem stems from improper protocol when it comes to tracking their weight. This is invaluable because you can't properly pursue a goal if you're missing part of the required information. It's like trying to put together a car without the blueprint, good luck because you're pretty much screwed!
At its core, I believe there are 3 basic principles one must follow in order to get an accurate picture of their weight trajectory:
1) Weigh yourself in the exact same situation every day.
Fat loss is a slow and sensitive process. Depending on their size/gender, individuals may aim to lose as little as 0.5% of their bodyweight per week which can equate to as little as 0.5 lbs for a 100 lb female! Food intake alone can result in day-to-day fluctuations in bodyweight of several pounds. Combine that with other variables such as time of day, water intake, etc. and you it's very easy to get an inaccurate and in way, erroneous weigh-in. To standardize this process, I highly recommend establishing a weigh-in routine and never deviating away from it. For most individuals I recommend weighing yourself on a digital scale (the same scale every day) every morning before any food/water intake, but after going to the bathroom. Something as little as forgetting to piss before doing a weigh-in can make it seem as if you are 1-2 lbs heavier than usual which can have a demoralizing effect on you if you're cutting.
2) Weigh yourself and record the value every day.
We as humans LOVE to make excuses in order to ignore any "setbacks" we encounter along the way. An example of this would be an individual not recording his/her weigh-in on a day when they surprisingly gained a few pounds because they believe that it's a temporary "anomaly". This can result from "cheating" on their diet the day before, increased sodium intake, or for no reason in particular and they just happen to gain weight that day. Regardless of which, always record the weight. Losing fat/gaining muscle is not a day-to-day phenomenon. It is a slow process which can only be accurately viewed and analyzed over a long period of time. In order to do this accurately, we need the full picture, not just your "good" days, otherwise the resulting weight data is selectively biased and inaccurate. This will prove essential for the 3rd and final principal below.
3) When comparing your weigh-ins over a period of time, use WEEKLY AVERAGES.
As I mentioned above, analyzing your weight trajectory is a long-term process. Your body fluctuates by several pounds every day due to numerous factors which are mostly out of your control. For example: perhaps you are cutting and a last week you weighed in at 180 lbs, but today you weighed in at 181 lbs. Theoretically this is possible even though you actually lost fat over the last week, but unfortunately it appears as if you gained weight! The 181 weigh-in may have just been a fluke or a "high day" whereas your weekly average weight is decreasing from the week prior, but you don't know that since you compare random individual days. I highly recommend not only weighing and tracking every day, but I also recommend averaging those values out every week and using the resulting averages as a basis of comparison. You may find that even though your weight fluctuates day-to-day, the resulting trajectory is towards your goal whether it be down for cutting or up for bulking. Sometimes clients tell me they are having trouble losing weight because the scale hasn't moved in the last few days or perhaps has even gone up on that day, when in reality they have lost 0.5-1.0 lbs over the course of the week according to their average weigh-in when compared to the week prior!
A very similar concept is the relationship between weather and climate as explained by Neil deGrasse Tyson in the following video clip:
Your daily weigh-ins are like weather (the dog in this video).
Your weekly average weigh-ins are like climate (Neil in this video).
It's not important what happens to your weight on a day-to-day basis.
The only thing that matters is your overall trajectory!