Ensure that you're eating enough calories during your window. If you don't, registered dietitian Leslie Langevin, MS, and author of The Anti-Inflammatory Kitchen Cookbook, explains that it can cause "the binge and starve cycle." If you don't eat enough on Monday and then on Tuesday you end up binging, and if this cycle continues, it can lead to weight gain. Being too hungry can totally disrupt your IF windows too (you don't want to dip under 1,200 calories daily). If you do, then intermittent fasting isn't to blame for your weight gain — it's just the fact that you're not eating enough and then eating too much. In this case, intermittent fasting might not be for you.
If you are caught in this cycle, Langevin suggests reducing your fasting window to maybe 12 or 14 hours. "This can still be helpful and reduce the time of eating in the day so the body is in calorie-burning mode more of the time, but still provides a longer window of eating."