If a process server is not able to serve documents, it is usually because the person has refused to accept them, was not home, or is deliberately avoiding being served. The consequences of this depend on the type of documents being served and the laws regarding service where you live.
In some cases, if it can be proven that you deliberately avoided being served after every acceptable method of service was used, legal action may be taken against you.
In other cases, if you refuse to accept documents, or if you were not available to accept them, and repeated attempts have been made by the process server, an application may be made to the court for an order to use an alternative or substitute method of service.