Check facts / ideas. This is wrong or at least doubtful.
Example: provide evidence to support this point.
Something left out. Poor proofreading means you don't spot careless errors.
Develop this idea, either through more detailed analysis or by further explanation.
Begin a new paragraph here.
Repeated word or idea. Vary your language. Take care with thesaurus!
Cohesion problem. The ideas within the paragraph / text do not link logically.