There are quite a lot you can read code is, but then they start again unable ZIP code is, do not know how code. This is very normal.
It's like you hear a song, out song, know this song or, but can not write a song like that. Or when reading poems, you know it or, you understand about it but can not compose poems similar.

I also had this problem for quite some time wondering, you can refer to here: https://daynhauhoc.com/t/hieu-tu-tuong-nhung-kho-code/37697

And his cause found 3 Main reason:

1. You do not understand a command, it does and when to use it. Do not understand, not knowing when to use this command, you do not know the code apply to all.

2. You do not know to solve this problem is to follow these steps, How does the order or did not even know what to do.

3. You have not much practice, or even too lazy to practice, leading to the first cause is not understood, and also lead to secondary causes 2 always.