The box may fit neatly beside the washing powder, but cats don't like Persil-flavoured biscuit. If your cat only eats small amounts of wet food, buy small tins or small foil trays. These are more expensive to buy, but there is less wastage. It isn't hungry enough to eat the whole serving. Reduce serving sizes rather than encouraging it to clean its plate and become overweight.
Some cats have digestive systems which only accept small amounts of food at a time.
Height of foodbowl. This is an odd one for most people. Some cats have problems eating from a bowl on the floor. Older cats and those with joint trouble have problems crouching down to eat their food. Hunting and scavenging.