The most important reason is that students often learn vocabulary out of context. Maybe they try to learn vocabulary by reading the dictionary. Or maybe they collect lists of words and memorize them. Some people buy vocabulary books. The problem with all of these methods is that you don’t learn vocabulary in context. You don’t see enough examples of the word in sentences. So you never really learn the correct way to use the word.

Research shows that the best way to learn new vocabulary is to read read read. Seeing vocabulary in a natural context – for example, in a newspaper or magazine article – will help you to use your new vocabulary correctly, and seeing it in context will also help you to remember it better.
So this is what you need to do. Read English books. Read English magazines. Read English newspapers. If you’re in an English-speaking country, read the advertisements on buses and signs on the streets and highways. Read English stories. Read graphs and tables and charts. Read the English operating manuals for your TV and washing machine.
When you see a new word, write it down. Write down the whole sentence because this will remind you of the correct context for the word. Write down two or three or four different sentences where you’ve seen the word. Write down the definition of the word.
Pay attention to the part of speech of the word: is it a verb, a noun, an adjective or an adverb? This is also a common mistake that students make; sometimes they choose the correct vocabulary word, but they use the incorrect part of speech. For example, they use a noun instead of a verb. They say “arrival” instead of “arrive.” Or “departure” instead of “depart.” So when you find a new word, make sure to learn its part of speech. And then write down every part of speech for that word. For example, if your new word is “protect”: protect is the verb form, protection is the noun form, protective is the adjective form, and protectively is the adverb form. Write down all four forms.
You can write your new words on a sheet of paper. Or you can write them on flashcards. You can even make flashcards online, on some online dictionary web sites.
In order to remember your new words better, try using them. Write some practice sentences and include some good context. Write a couple of sentences. Ask your English teacher or any English speaker to check your sentences. Do they make sense? Are they good sentences?
When you learn vocabulary this way, you learn to use it correctly and naturally. It is the best way to learn new vocabulary