After 3 weeks, you should be using some nitrogen.
Most common issues that lead to this type of stunted growth is overwatering and light distance.
Overwatering slows root growth as they don't receive the oxygen they need. This will slow overall growth.
Too much light can stress her and instead of stretching like they do with too little light, they stop upwards growth to avoid being burned.
Almost guaranteed you are suffering the consequences of one or both of these issues.
If you need more info on light distance and where you should be, shoot me a message.