Hard water is the most common cause
Mineral deposits from hard water leave the chalky white film you see on glasses and dark dishware. A rinse aid dispenser (most dishwashers have one built in) makes a significant difference by helping water sheet off dishes instead of forming droplets that dry into spots.
Check the rinse aid dispenser
If it's empty, that alone is often the entire problem. Refill it and see if the next few loads improve.
A clogged or blocked spray arm
Mineral buildup or food debris can clog the small holes in the spray arms, leading to uneven cleaning and rinsing. Remove the spray arms (most simply lift or twist off) and clear the holes with a toothpick, then rinse under running water.
Water temperature
Dishwashers rely on hot water to dissolve detergent fully and rinse effectively. If your water heater is set too low, or the dishwasher runs before the hot water has reached the kitchen, spots and residue are more likely. Running the kitchen faucet until it's hot before starting the dishwasher can help.
Overloading or using the wrong detergent
Overcrowding blocks water and detergent from reaching every surface. Using regular dish soap instead of dishwasher-specific detergent will also cause serious sudsing and residue problems.
Keep appliance maintenance on a real schedule
Hiveix reminds you before small issues become bigger ones.