Coming in short is almost always a content problem rather than a length problem. The fix is another claim or deeper analysis of evidence you already have — not longer sentences, throat-clearing transitions, or a paragraph restating the introduction. Markers recognise padding instantly and it reads as having nothing left to say.
Coming in long is a better problem, because cutting improves almost every draft. Start with background in the introduction, then quotations longer than they need to be, then any sentence that explains what you are about to do rather than doing it. Most first drafts lose 10% without losing an idea.
Check whether your word count includes footnotes, references and appendices — conventions differ, and the penalty for exceeding a limit is usually mechanical.