While there are many reasons why someone may struggle with their progress (e.g. nutrition, illness, fatigue, etc.), one common reason is the intensity at which someone decides they need exercise to meet their goal(s).
Different outcomes such as strength, flexibility, cardio and aesthetics require different methods to achieve success. Positive changes in our body result from an adaptation to a stressor or improved efficiency to cope with a given stress. The stressors we must provide our body come in different forms, such as elevating the heart rate, increasing the ROM, increasing the load (weight) we move, or providing a caloric deficit (burning more calories than you eat).
The challenge for many people comes with the reluctance to endure the discomfort or pain associated with pushing the limits of these stressors. However, only when we push beyond our current levels will the body positively adapt and allow us to reach our fitness goals.