My dishwaser does not clean very well on the top rack. It is about 10 years old

Many items can cause a washability problem. The water level in the unit,the type of soap,the way the items are placed in the dishwasher etc. If the soap is the same and the way you put the dishes in is the same then it must be the machine itself. After 10 years of use it is possible the pump is getting tired and does not circulate the water as well as before. This will definitely cause a cleaning problem. It could also be debris down in the lower area of the dishwasher blocking the flow of water. You can check this on most units by removing the lower rack and seeing if you can remove the strainer to see if any debris is present.