How big of a difference do you mean when you say you are getting worse? Were you consistently scoring MUCH better before? If so, go back and assess what you were doing then that you are not doing anymore. Did you change your strategy?
If your scores were never that great to begin with, you need to do better analysis of your incorrect answers even from the beginning. Try to notice a pattern in what you are having difficulty with. Is it a particular type of question you are always getting wrong? Is it a particular type of passage? Are you running out of time? There is no formula to getting a 15 on Verbal, so you have to find what works best for you. Don't go easy on yourself by letting incorrect answers go. Dissect it, because you got it wrong for a reason.
If you don't understand many of the basic words in the passages, I would recommend postponing the exam and working on improving your English first. If it's with more uncommon, complicated words then don't worry, you just need to learn how to use context clues from the sentences better and not get intimidated by convoluted language. English is my first language, but I still don't understand some of the complex vocabulary, and it hasn't affected my scores because I can still gather main ideas, and understand words/phrases like "however", "yet", "it seems as though", etc. which are often the most important. Overall, my biggest advice to you is to practice, practice, practice and do heavy analogy while doing so. Good luck. 👍