The reason why kids often don't get maths, specifically algebra, is because they try to learn it sequentially. This is no fault of theirs, since it is mostly taught that way. Start with this topic, then go onto another, and so on, day after day, topic after topic, and perhaps if you are lucky, you fit it together in your head and it works.

Luckily, it is easier than that. I discovered early on in my teaching experience that it was better to teach algebra altogether in one lesson. I call the technique "on-yer-bike" because the first class I tried it with, I used the metaphor of learning to cycle. No-one learns to ride a bike through a book. Maths is the same. It's not about what is written down, but what one does. It's not complicated. In the same way you don't need to know about gyroscopes in order to ride a bike (without which riding a bike is almost impossible), so you don't have know how it works. It just does. And learning maths is like that too. Or juggling. It's about doing several things at the same time. Once you get the trick of it, it becomes easy.

Once you get the hang of it, you can end up doing very high levels of algebra, learning tricks and stunts as you go, from using brackets to completing the square.