My score does not increase for CR/WR section! PLEASE HELP!

My CR score and WR score just does not increase. My current score is CR 600, WR 620 (Essay 9)
It was pretty easy to increase up to 600s. When I first did the diagnostic SAT test 6 months ago my CR score was 420 and WR was 450. I memorized a bunch of vocabulary from random books (I didn’t decide one particular book, I just wandered around within various vocab books) and then I think that helped me a lot to increase until 600.
But, I have NO IDEA how to increase up to 700s. I actually achieved 600 in practice test 3 months ago and it just didn’t increase past 3 months.
Also, I just studied basic grammar rules from various books and sparknotes, and I achieved 620 in practice test 3 months ago and it just didn’t increase past 2 months.
Can anyone help me here to increase CR/WR up to 700s score range…? It is a really challenging task…