Some information isn’t meant to be remembered. Data — a phone number, for instance — is held in short-term memory for just a minute or so. But when you reinforce information (say, by dialing that phone number every day), a long-term memory forms.

When you have trouble remembering something, it’s often for one of these reasons.

Insufficient attention: The information never made it into your memory bank. This typically happens if you’re distracted at the time an event takes place (you’re thinking about what to make for dinner when you put your gloves on the counter). “You can’t recall something that never actually made it into your memory system in the first place,” says neuropsychologist Aaron P. Nelson.

Interference: This occurs when old and new memories overlap. For example, you’re trying to remember Meryl Streep’s last movie, but all that comes to mind is Sophie’s Choice.

Fading: If you don’t revisit certain information, such as dates, names, and appointments, it tends to vanish.