You should provide variety. A succession of loose, short sentences is annoying. Short sentences are not always bad and can often be effective, but an entire paper of such sentences shows lack of effort or inexperience. Develop appropriate compound and complex sentences for your paper. If you read one of your long, complex sentences and it seems to make no sense, syntax is probably the problem.
Syntax refers to the way the words are put together to construct a sentence.
Often syntax is poor when the subject is separated from the predicate by a long series of phrases or modifiers. Another problem of syntax is misplaced modifiers. The modifying phrases are so far away from the words they modify that the sentence sense becomes murky. When doing so, the antecedent is often unclear. This was a difficult project.