The United States is strongly committed to democratic values. However, democratic values sometimes come into conflict with one another. Therefore, it can at times seem that the US is less than democratic to some people.
Democracy is based on a number of values. The most important of these are popular sovereignty (rule of the majority), political liberty, political equality, and personal liberty. The US is strongly committed to all of these. However, sometimes two values come into conflict.
For example, the US believes in personal liberty. It believes that people should have the right to act as they please in their personal lives. However, it still bans gay marriage. This would appear to contradict the idea that the US is democratic. But part of the reason the US bans gay marriage on the national level is that the majority of the people still want this. In this way, the US violates personal liberty because it is committed to popular sovereignty.
The US, then, is strongly committed to democratic values. However, these values sometimes conflict, leading to the impression the US is less than democratic.