The problem is that retrieval of information from a busy, cluttered memory is not always easy. The context in which we encode things can play a role in how easy it is to remember them, with items being harder to retrieve or recognize in an unfamiliar setting (most people have experienced this with faces, e.g. seeing a neighbour when abroad on holiday). Similarly, even when we really should know a word or name, and we know that we know it, sometimes it doesn't quite come, at least not straight away. The concept of something being on the 'tip of your tongue' goes across cultures and has been around for hundreds of years.
|It is a common experience that a memory|
is 'on the tip of your tongue'.
It is relatively easy to invoke the tip of the tongue (TOT) phenomenon - giving a person a list of 10 definitions of rare words will usually bring about the experience at least once. Brown & McNeill (1966) researched the phenomenon, stating that a participant "would appear to be in mild torment, something like on the brink of a sneeze, and if he found the word his relief was considerable". The researchers also found that giving a cue such as the first letter of a word was often enough to 'trigger' the memory.
TOT occurs roughly twice a week in most adults (Schacter, 2001). It is thought to be uncommon in young children - although I don't know of any research into 3-4 year olds, my experience as a parent suggests that children at that age often struggle to find the right word but are more willing than adults to substitute a different word.
One more thing - lots of research into memory uses words - here is an interesting test into memory and retrieval for faces.
Brown, R. and McNeill, D. (1966). The "tip-of-the-tongue" phenomenon. Journal of Verbal Learning and Verbal Behavior, 5, 325-337.
Schacter, D. L. (2001). The Seven Sins of Memory: How the Mind Forgets and Remembers. Boston: Houghton Mifflin.