While the mentors try their best to answer all the queries from the students, sometimes due to the load of the queries they may tend to miss a few.To ensure that your query is replied, make sure to,
2. Indicate (through action than just words) that you are a strong candidate willing to learn and join the team.3. Avoid asking questions that are well-covered in locations easy to find (such as FAQ).
While we tend to encourage all the questions, vague questions may give a signal to the mentors that you are not serious about your project, and probably too lazy to find the information yourself. I have received questions such as "Please let me know how to start". While this sounds a valid question, this probably is already covered in the project descriptions and discussed in the mailing lists, irc, etc. So you should rather inform the mentors what you have done, and how you expect them to help.
If you decide to contact them through private email, make sure to improve your question (as suggested in step#2). If your question is of the same nature as "Please help me start" with no reasonable input, it may still be ignored. While this sounds like arrogance, not every mentor has enough cycles to devote during the application period. Also a few of the mentors emails from around 50 - 100 students. Not all of the students are going to end up in the GSoC (some do not even apply to the project after querying about that) and a lot of them just do not give any decent attempt by themselves at all.