All designers have concepts or designs that have ended up on the cutting room floor. The reasons vary; maybe a client didn’t like the color you chose to work with. But, sometimes, the reasons the design didn’t get chosen is perfectly reasonable.
Maybe you didn’t understand what the client wants. Some clients aren’t looking for a complete overhaul. If a client wants a small area or one aspect of a page changed, don’t let yourself run away with your concept. It is also possible to just go too far technologically. I think every designer has come up with some “masterpiece” only to realize it is not entirely feasible.
There are lots of different reasons a design can not be selected. Issac Pinnock from Made By Many has an article where he pulls out specific examples from his portfolio that never made it to publish. The main reason is usually the same, however. The designs don’t give the client what they wanted.
Understand your client, and ask plenty of questions to make sure you know what they want. That is the secret to making a design that gets chosen.