The most common reason a student may have a high similarity score is that they may be using too many direct quotations in their work, because every single word in a direct quotation will be a match this will naturally cause you to have a higher similarity score. There will be times where a direct quote may be suitable but they should not be overused and the alternative to direct quotes is paraphrasing (see also FAQ below).
If you have paraphrased but still feel your score is too high, this may mean you are not changing the wording of a direct quote as much as you should be.
Please Note: Having a high similarity score does not mean you have plagiarised, but it may suggest that your academic writing might be improved by better paraphrasing. If you have concerns with your similarity score you can always contact the Library Academic Support Centre.