The detergent you are using may be insufficient or poor in quality. You can try increasing the detergent dose. The program you have selected may be incorrect, you should select a heavier wash program.
The stainless steel may be of a lower quality, you can wash them by hand periodically. The detergent may be thrown over cutlery and saucepans - for this, ensure that the detergent is loaded into the detergent dispenser correctly.
You can wash most good quality plastics in dishwashers. However, do not force them into the racks or they will change shape. Do not put items that soften at temperatures of 50° C and over into the dishwasher. Also, do not wash small plastic items in the dishwasher as they may fall onto the heating element.
When it refers to 12 place settings it means a setting for 12 people. ie. 12 dinner plates, bread plates, saucers, cups and cutlery.