Almost everybody has had the feeling of knowing the answer to a question, but not being quite able to say it. this is called the "tip-of-the-tongue" phenomenon, and is a failure of:

Retrieval failure is the point where the data is in long term memory, yet can't be gotten to the exact event. The powerlessness to recover a memory is common amongst the most widely recognized reasons for overlooking. Retrieval failure is the inability to review a memory because of missing jolts or signals that were available at the time the memory was encoded.