Proactive interference
Proactive interference is when an old memory disrups a new one. This makes it hard to recall the memory if you are able to recall it at all. We do not forget these memories the other ones just get in the way of finding them. For example if you were to learn about a word in Health Sciences, and you know that word very well. In a few months you may not remember some of the more recently learned words because this definition is getting in the way of you learning these new ones.