As a GRE tutor, I work with a lot of people who have trouble in quant. They’re usually a little embarrassed and somewhat baffled about this.
They say, “I haven’t done any of this stuff since high school,” or “I can’t believe I forgot this.”
Here’s what I always tell them: it’s not that you forgot the math you learned in high school, it’s that you never learned quant properly in the first place.
High school tests math in a really shallow and broad manner. You learn a lot of formulas, and a few problems to apply each formula to. The GRE, on the other hand, tests math in a narrow and deep way. You learn a few formulas, and a lot of problems to apply each formula to.
That means that unlike what you were taught in high school, your task on the GRE is not to learn a bunch of formulas. This is exactly where most students (and prep courses) stop, though, and then they’re surprised that their GRE score doesn’t improve.
Your task on the GRE is to be able to see a new problem, recognize the strategy you need to employ, then progress smoothly on the problem until you get the right answer. So, how does that change how you prepare?

Instead of learning a bunch of formulas, do a lot of problems and get a lot of problems wrong. Each problem you get wrong is another chance to learn.

You need to review and learn from the questions you get wrong. Don’t just churn through questions, but actually use the questions you get wrong as a learning experience. An error log can help a ton with this, either in spreadsheet or app form.

Test and refine your strategies on practice tests. If you can’t get a good score on a practice test, you will not get a good score on the real test. And again, once you take a practice test, learn from it with an error log!
tl;dr: you don’t improve because you don’t do enough questions, and you don’t learn from the questions you do.
My credentials for this post: 170V/166Q GRE, full time GRE tutor, writer of strategy guides and really long essays on how to learn.