It's called a Plateau. So basically any series of exercises or diets you do apply a "stress" to your body, which it adapts to in many ways. For instance, running (yea sorry I run - among other sports). If you run 1 mile a day, 5 days a week, in say 8 minutes, your body will adapt until it is as good as it needs to be to run that. It will then stop adapting. The only way to improve the fitness in that case would be to either try to run that mile faster, or run more than a mile.
The same is true of dieting, if you say reduce your calorie intake by 10%, your body will begin to burn fat as well as attempt to make its own physiological processes more efficient - note a reduction in fat cells and muscle cells reduces the number of calories your body function. So as time goes on you will plateau there as well.
In general neither of these solutions will work in a vacuum. You need to have a combination of both. In addition I wouldnt recommend cutting calories too much as that will cause your body to basically go into emergency mode, where it will use as little energy as possible. You'll feel fatigued, tired, weaker, your core body temperature will drop a little, all resulting in you burning less calories per day.
But if you've hit a plateau, its time for a change, to add something, another stress your body will need to adapt to. In general early in a fitness program of some kind, this would usually be met by adding more distance to what you're already doing (no more than 5-10% a week of total weekly volume). So if you're simply dieting then I'd add in some walking or jogging or biking or elliptical, something to boost the metabolism and burn calories. If you're hitting the gym like a champ cant imagine going ANY longer, then maybe you're actually going too hard. Being able to go 80% for 3 days beats being able to go 100% for one day, 0% the next day. Maybe you need to switch up exercises, so elliptical one day, bike the next.
But I agree, if you could provide more details it'll be easier to come up with an exact recommendation