Almost always because your jaw has changed shape, not because the denture is defective. The ridge of bone that a denture rests on slowly shrinks once the teeth are gone. It is a normal process and it never fully stops, so a denture that fit perfectly when it was made is sitting on a different shape a few years later.
How fast that happens varies a lot between people, and it is faster in the lower jaw, which is part of why lower dentures give people the most trouble.
There are two ways to deal with it. A reline reshapes the fitting surface of the denture to match the ridge you have now, which is the usual first answer and it is not a new denture. If the ridge has shrunk far enough that relines stop holding you for long, implants are the other option, because they anchor the denture to bone instead of resting on a surface that keeps changing.
Bring the denture in and let me look at it. Whether you need a reline, a new denture, or implants underneath it is not something you can tell from how it feels.