This could be a result of many different things. Poor mental or physical health could lead to an individual not being able to concentrate properly, and this inability to concentrate can lead people to have difficulty retaining detailed memories.
Medicine
Many types of medicine involve the altering of certain brain chemicals, and as a result, certain medicines can confuse and cause memory problems for people.
Age
The idea that older people tend to develop memory problems seems to be common knowledge, but not many bother to ask why. The decay theory serves as an explanation for the deterioration of the memory function in individuals.
Dementia
Dementia is a condition of the brain that can lead to the gradual loss of mental ability. There are a few types of dementia, but Alzheimer's disease is by far the most common.