Fedaykin's right. Changes in actual body fat aren't linear, and do not happen as fast as the daily fluctuations in transient mass - water, glycogen, foods that are still being digested. If you're not gaining or losing very quickly, it can be hard to tell if you're gaining or losing at all, in fact.
Here's a chart of what roughly 4 pounds in weight gain looks like, if you take two months to do it:
The black trendline that Excel added is pretty much my "real" weight. The blue line is what the scale says. They don't intersect all that often. Still, you can see that the peaks and valleys increased at the same rate.
It's frustrating, but the scale is not a reliable indicator of progress or body fat. Unfortunately, patience is needed to watch the long-term trends.