WHY DOES THIS HAPPEN
Weak, sparse growth on your plant can be the result of a number of different issues, some more severe than others.
For instance, it may be a symptom that your plant isn't getting enough light, in which case it will not have enough energy to grow healthily, and may become stretched / lopsided as it tries to find a light source.
Alternatively, weak growth can be a sign of an imbalance in fertilizing - both overfertilizing and underfertilizing can lead to the development of this symptom. If you're unsure which of these is the cause, try to look out for other symptoms and work by a process of elimination, as it's likely your plant's overall health will be affected by this imbalance.
Finally, it may just be a sign that your plant needs to be repotted! Sometimes a plant may not be growing well because it simply doesn't have enough space to spread out.
You can often tell if this is the case by checking the roots - if they're pressed up against the edges of the plant pot, or even poking out of the drainage holes at the bottom, then this is a sure sign that it's time to upgrade to a bigger pot.