DIAGNOSIS This usually happens because the cup wasn't emptied cleanly. A piece of debris—like a twist tie, a chunk of cat litter, or a large crumb—is stuck in the cup's guide channel or is blocking the latch mechanism at the bottom.
FIX Take the cup out again. Look at the bottom of the cup where the gray plastic latch is. Clear out any debris there. Then, look inside the empty cavity on the vacuum where the cup sits. Shine a light and check for any foreign objects, especially around the small metal electrical contacts at the front (these tell the vacuum a cup is installed). Remove anything you see.
IF THAT DIDN'T WORK Check the plastic rails on the sides of the dust cup. Make sure they aren't cracked or deformed. Also, ensure the cup is oriented correctly—the handle should be facing you, and the latch at the bottom should be pointing forward toward the front of the vacuum.
HEADS UP Don't force it. If it's not clicking in with moderate pressure, something is blocking it. Forcing it can break the latch on the cup or the receiver on the vacuum, and neither of those are fun to replace.