Have you gone through what you got wrong in the test? If you're getting high 70s, you obviously understand the content very well and remember most of it.
If you find you were just forgetting a few things, maybe do more testing to ensure you get perfect answers. Seneca Learning (exam board specific) or Quizlet (fun little quizzes) can be good for this
If you got a few areas wrong - ie your understanding wasn't quite right - maybe try and look up different explanations to see if it helps make it clearer. Youtube channels can be good - freesciencelessons, primrose kitten, science with hazel etc - and Seneca Learning also has explanations which go well..
If you just don't get full marks in a lot of the questions, especially the longer one, it will probably be a matter of exam technique. try and do more practice of past paper questions, and keep going really thoroughly through the markscheme. With Biology, a lot of it is literally using the word they want so learn all of the nuances!