If you have received feedback that your writing needs to be more academic, it probably means one of several things.
1. Your writing may not use enough citation & referencing.
Your claims need to be supported by suitable references and evidence.
Further reading: Referencing
2. Your writing may be too certain in its claims.
Make sure you are using suitable hedging/ academic caution to protect your claims, and avoid definitive statements or generalisations.
Further reading: Cautious Language and Hedging
3. Your writing may need to express more criticality.
Academic writing aims to explore and evaluate concepts and research findings, from the point of view of adding to our knowledge of the area.
Further reading: Criticality
4. Your writing may be aimed too clearly at a professional audience.
Many postgraduate students are competent writers in a professional sphere, but as a student research writer you will need to use a different tone.
As mentioned above, the aim of your writing at the Institute will often be to explore and evaluate concepts and research findings from the point of view of adding to our knowledge of this area, as a research community. This can be different from a professional context, where the aim may be to inform the audience or to recommend certain actions.
You may find that you need to write in a way that feels less authoritative and less practical than you are used to. Notice the tone of the research papers you read, and the way they interact with knowledge claims. Your reading is often your best writing teacher.
5. Your writing may be too colloquial (too casual).
Make sure you are using formal language, and avoid colloquial phrases such as 'every coin has two sides', or 'they've been left on the scrap heap'.
Take the lead from the texts you are reading, and use similar vocabulary and phrasing.
Compare the following two short texts, (A) and (B). How many differences do you see in the second text? What is the function/effect/purpose of each difference?
You will probably notice that (B) is more 'academic', but it is important to understand why.
(A) Extensive reading helps students to improve their vocabulary.
(B) Research conducted by Yen (2005) appears to indicate that, for a significant proportion of students, extensive reading may contribute to an improvement in their active vocabulary. Yen's (2005) study involved learners aged 15-16 in the UK, although it may be applicable to other groups. However, the study involved an opt-in sample, which means that the sample students may have been more 'keen', or more involved in reading already. It would be useful to see whether the findings differ in a wider sample.
(Please note that Yen (2005) is a fictional reference used only as an example).