Improve SAT writing score?

Hello, I’ve a small problem & I hope someone has a solution for it.
I’ve taken 3 old sat tests & by chance my writing scores are pretty much the same 540-550-520… Even though I studied hard for the new SAT writing tests, I got 310 which is, according to the concordance table, equals 550 in the old SAT. I’ve got decent scores for the other subjects 720 CR score & 760 for maths… I really need to improve my writing score; I used many books and many guides, but never sensed an improvement, even when I tried to stimulate myself taking the test… So I hope anyone can give me an advise.

But reading and writing are a single score in the new SAT, how did you convert writing alone? anyway, don’t take old SATs because they are very different from the new ones. Now, which types of questions do you tend to get wrong? vocabulary? sentence insertion? punctuation? you need to know where are your weak spots if you want to improve them.

They’re one score, but if you get to look deliberately in the score details, you can see how much you scored on writing & on reading… and the type of questions I’m usually getting wrong are those related to the conventional expressions and idioms which are like 1 or 2 per passage, the topic/introductory sentences in the begging of the paragraph, and those type of questions which ask you whether or not the author is correct about putting this paragraph/sentence in it’s location