I have two thoughts.
Personally, I'd be a mess if I had to eat in a cafeteria or restaurant on a regular basis, as I'm very sensitive to food additives. For example--those salads. Many commercial food sources will sprinkle their greens with MSG to keep them fresh, and I 'react' to MSG quite dramatically. However, even if you don't have a dramatic reaction, if you're at all sensitive to such things, you can retain water and retard fat loss. So it may be the commercial food you're eating that's the problem.
Here's what I'd try--bringing my own food for a week just to see if there's any difference.
Also, are you weighing daily? It's possible that what you see as loss only on weekends is actually your loss for the week. The scale is deceptive because as we lose, the body tries to retain homeostasis, and rarely will it show actual fat loss on a daily basis. Throughout my weight loss (almost 200 lbs), I weighed only weekly because of this--and I also kept a record of that weekly weight so that I could see long-term trends.