There are going to be clients who are unhappy from time to time. Communication is partly to blame. Without effective communication, the project isn't going to be completed to the client's expectations.
Don't be shy - ask questions!
If a client's instructions are unclear to you, ask them to be a little more specific so that you can complete exactly what needs to be done. You can also type up some interview questions within your gigs so that your clients can answer these specific questions that will tailor their project to their expectations. For example, ask your client if their article is a B2B (business-to-business) or B2C (business-to-consumer). Also asking your client who the target audience will be and what target keywords they want incorporated into the article will really help you meet their expectations.
Communication is everything. Reach out and dig for specifics. The more information you get from your clients, the easier your job will be to meet their expectations.