You mean why don't people lose weight? Well, it can genetics or maybe you are not eating the right things or exercising correctly. Cutting calories doesn't mean skip breakfast and eat a big dinner and lunch. Working out doesn't mean working out for 10 minutes. You have to cut back on sugars, caffeine, dairy and white flour. If you are going to work out, running is a great exercise. Good luck!
Usually they are allowing themselves too many calories, even though counting them, or over-estimating their workout. They should redo the math and be honest with themselves about their eating/exercise routine.
It depends on a lot of factors, such as: How long have you been keeping this diet & exercise regimen? How many calories/day are you eating? How much/what types of exercise are you doing?
To lose weight quickly, try consuming 1,000 calories per day, don't eat ANY fatty foods/junk food, drink lots of water, eat plenty of raw veggies & go for daily walks -- walk or run for at least 30 minutes/day. If you're still not losing weight, see your doctor, you may have a hypo-active thyroid, or other health condition. FYI: Consuming 1,000 calories or less per day should ONLY be done SHORT-TERM; only while you're losing weight. Once you reach your ideal weight, go up to 2,000 - 2,500 calories per day & keep eating a healthy diet that's low in fat, & continue with an exercise routine. If you don't, you'll gain back any weight you lost! :)