First, Your rinse dispenser only releases a few drops of rinse agent (approximately 1/4 tsp)during a normal wash cycle. Depending on usage you may not see a major decline in the dispenser window.

On the other hand, you could have a defect in the dispenser switch and linkage, or the dispenser outlet could be clogged with old rinse agent and also will not dispense.

To access the rinse agent dispenser and components you will need to dismantle the inner door panel (plastic liner) from the outer door panel (metal panel)

If you do not feel comfortable dismantling the door you may want to contact a servicer to diagnose and repair your unit.

The opening for the rinse aid is under the fill cap. the small hole or slot just under the cap on the bottom side of the opening.

In most cases servicers would do a live 120 volt test at the switch to see if it worked and activated the dispenser and opened to release the rinse agent.

I would not reccomend you perform that test, you could hurt yourself.
If you do not find any debris or residue in the dispenser opening, I reccomend you have a servicer out to diagnose and repair your dispenser.