Somewhere around band 6.5 many students decide the way up is bigger words, and their scores stop moving. The lexical criterion rewards precise and natural use, and marks down words used awkwardly — so a rare word in the wrong collocation costs more than the common word it replaced would have.
Three habits separate the students who get past that plateau.
Collocation over synonym. Words travel in company. You conduct research, you do not make research. Crime is committed, problems are tackled, an opportunity is seized, a policy is implemented. When you learn a word, learn the two or three words that sit beside it, and never take a synonym from a list without checking what it is normally used with.
Specific nouns. "Things", "stuff", "aspects" and "factors" are where an argument goes vague. Name the thing: not "environmental aspects" but "air quality", not "economic factors" but "wage growth". Precision reads as sophistication because it is sophistication.
Repetition is allowed. Trying never to repeat a key noun produces the strangest sentences in IELTS — an essay about cars that has to call them "automobiles", then "these vehicles", then "such transportation devices". Repeat the accurate word. Vary the sentence around it.
And be careful with the linking words. Moreover, furthermore, in addition and however are fine in moderation; stacked at the start of every sentence they read as assembled rather than written, and the descriptors name exactly that.